DTU ProjectLab - User contributions [en-gb]

The Critical Chain Method

2015-09-28T23:19:42Z

F.labrini:

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
* If there are any problems in finding the article with the excel list, is because I changed the title.

The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. Identify project’s constraint===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. Decide how to exploit the project’s constraints===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. Subordinate everything else to the above decision===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. Elevate the project’s constraints ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. If, as a result of the previous steps, the constraint has alleviated, return to Step 1 ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

Now, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.

Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation in some cases.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention. All leading to respect the end due date.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM, is the indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known [https://en.wikipedia.org/wiki/NP-hardness NP-hard problem]<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules are used, which generate solution that are hoped to be close to optimum<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that wants to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T23:10:50Z

F.labrini: /* TOC to Project Management */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. Identify project’s constraint===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. Decide how to exploit the project’s constraints===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. Subordinate everything else to the above decision===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. Elevate the project’s constraints ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. If, as a result of the previous steps, the constraint has alleviated, return to Step 1 ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

Now, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.

Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation in some cases.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention. All leading to respect the end due date.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM, is the indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known [https://en.wikipedia.org/wiki/NP-hardness NP-hard problem]<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules are used, which generate solution that are hoped to be close to optimum<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that wants to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T23:05:16Z

F.labrini: /* Limitations */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

Now, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.

Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation in some cases.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention. All leading to respect the end due date.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM, is the indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known [https://en.wikipedia.org/wiki/NP-hardness NP-hard problem]<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules are used, which generate solution that are hoped to be close to optimum<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that wants to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T23:00:09Z

F.labrini: /* Benefits */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

Now, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.

Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation in some cases.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention. All leading to respect the end due date.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:59:15Z

F.labrini: /* Benefits */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

Now, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.

Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation in some cases.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:57:09Z

F.labrini: /* Planning */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

Now, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.

Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:54:15Z

F.labrini: /* Planning */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
1. the first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.

2. the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>.

3. the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.

4. this step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:48:49Z

F.labrini: /* 3. Subordinate everything else to the above decision */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:47:30Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency equals the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:46:19Z

F.labrini: /* TOC to Project Management */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

This section presents the TOC's steps executed in Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:43:34Z

F.labrini: /* Theory of Constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind it.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:42:23Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]] addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 4 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.
Traditional project scheduling methods and Monte Carlo analysis, estimate the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:37:15Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contributes to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. People make considerable provision of safety time in order to make commitments they could meet with high expextancy.
In addition, managers tipically selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Talk:The Critical Chain Method

2015-09-28T22:33:05Z

F.labrini:

Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/different in the article.

''Thanks for the feedback. However, after I talked with Joseph I decide to focus on just one topic. It could be a future topic to investigate.''

==Reviewer 1, s112960==

'''Overall'''
* It looks to me as if you are following the ‘method’ structure
* Clearly the topic is related to a project, program or portfolio topic
* It’s written very clear and straightforward and is thus easy to read
* It’s an interesting topic.
* Even though the article is not done I like how it’s build up e.g. how ‘limitations’ of traditional approaches leads to the explanation of Critical Chain Method
* It’s good that you have many different references both books and online. This makes your article trustworthier.

'''Abstract'''
* Good explanation of why CCPM was made (response to projects resulted in larger duration etc.)
* If it is possible, it would be awesome if exactly how CCPM differs from traditional methodology is explained in one sentence so we already know in the beginning. You write it differs in how uncertainty is handled – but how exactly?
''Nice comment, but the sentence was just present in the draft ;)''

'''Theory of constraints:'''
* Nice illustration. However, it will make it easier to read it, if you made it bigger.
''Thanks for the tips. I am thinking that in order to avoid making the article too long, some of the figures will remain of the same size. To better visualize them it possible to click on it.''
* Good use of a quote. You might want to consider mentioning the one you are quoting by name e.g. ‘As explained by Goldratt…’

'''Undesired effects of traditional approach:'''
* I like the mentioning of a ‘’Student Syndrome’’
* Again, good use of a figure, but it needs to be bigger
''Thanks again. See previous comment''

'''Suggestions for improvement:'''
* Remember the bibliography of references
* It might be an idea to have examples in the article. It would also be interesting in comparing Critical Path to CCPM.
''Due to space limitation, I choose to deeply analyze the CCPM and I add specific explanation step by step with an external example.''

I’m looking forward to reading your finished article. Good luck!

==Review3, s150794:==

'''Overall:'''
* My first impression of this article is that it is clear and straightforward, and looks good with clearly defined headings and sections, and there are descriptive pictures with explanation.
* You have a good academic language throughout the article.
* The article is a bit short, but I see that you have some more topics you want to write about. :)
'''Abstract:'''
* A good introduction to the term, but I'm not sure if you need the first sentence.
'' I forgot to remove it.''
'''Theory of constraints:'''
* I like that you start with a short introduction of this topic.
* It think it is a smart way to highlight the implementation how you use the 5 points from the first paragraph to the next, with an adequate explanation.

'''Undesired effects of traditional approach:'''
* Looks good so far. I look forward to the rest 
'''Some suggestions for improvement:'''
* There are some space between some sentences that may give the text less flow. Tips could be to make the first part of “Theory of construction” to make it to a coherent paragraph.

''I remove the spaces where unnecessary, but I did not get the second part of your sentence.''
* Some sentences that where a bit long, which made it a bit hard to follow. A tip is to either divide of rewrite the sentences. Two examples:
** “CCPM is an outgrowth of the Theory of Constraints(TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, “Critical Chain”[2] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.”
** “TOC is a systems-management philosophy, originally applied to production system, based on the principle that any system must have a constraint that limits its output.”
'' Thanks for noting this. It is an "italian problem" I am trying to avoid. I shortened many sentences''
* Some of the sentence had some grammar faults. I would looked a little on sentence structure.
'' Thanks. Hope I found all the grammar mistakes''
* In addition to CCPM you could include a paragraph on risk, since you have this in the title.

''It would be a really nice idea. Since risk is a quite wide topic, I am not sure if it would fit in my article.''

==Reviewer 2, s145170:==

'''Overall:'''
* The article clearly follows the “methods” structure, states the topic from the beginning and is compact and to the point.
* The topic is related to a project, program or portfolio topic
* Good and clear academic language
* Figures that correspond to the theory described and cover the main aspects of the topic
* The transition from the one headline to the other, as well as from sentence to sentence is logical

'''Abstract:'''
* Good description of the context that is directly related to the Method
* Important that you mention the Critical Path Method, which differs from the CCPM and you will compare them

'''Theory of constraints:'''
* Nice introduction to the topic by stating the relation between CCPM and TOC, which explains why you will analyze the second.
* Even if you describe the implementation of the “five focusing steps” to Project Management, you could refer to a real-world example, which would be combined with the analysis of the theory that you already did.
'' Thanks for the tip. I add a practical explanation regarding PM along the five focusing steps.''

'''Undesired effects of traditional approach:'''
* Important that you mention the undesired effects of traditional approaches, so that you strengthen the implementation of yours.
* I like the example with the student syndrome, as it helps understand what you describe in practice.

'''Suggestions of improvements'''
* As you cover the theory adequately (even if a part is missing, it is clear from the contents), a practical example is needed, maybe in the form of small practical implementations/examples like the “Student Syndrome”, in order to reinforce your theory description.
* You could also add your own opinion somewhere in the text. As it is difficult to include our own opinion in the description of a theory, you could evaluate the implementation of this theory in a small example, as I proposed one point before
''Thanks, I change a bit the structure and add pratical instruction both about five steps and about CCPM method itself.''
* Maybe you could avoid some voids between small sentences.
* If the figures are not yours, you may have to add a reference to avoid copyright problems
''I had the source in the description of the figures, not sure if it is the right place''

Very interesting topic with nice compact structure! I wish you good luck with the rest!

Talk:The Critical Chain Method

2015-09-28T22:32:47Z

F.labrini:

Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/different in the article.
''Thanks for the feedback. However, after I talked with Joseph I decide to focus on just one topic. It could be a future topic to investigate.''

==Reviewer 1, s112960==

'''Overall'''
* It looks to me as if you are following the ‘method’ structure
* Clearly the topic is related to a project, program or portfolio topic
* It’s written very clear and straightforward and is thus easy to read
* It’s an interesting topic.
* Even though the article is not done I like how it’s build up e.g. how ‘limitations’ of traditional approaches leads to the explanation of Critical Chain Method
* It’s good that you have many different references both books and online. This makes your article trustworthier.

'''Abstract'''
* Good explanation of why CCPM was made (response to projects resulted in larger duration etc.)
* If it is possible, it would be awesome if exactly how CCPM differs from traditional methodology is explained in one sentence so we already know in the beginning. You write it differs in how uncertainty is handled – but how exactly?
''Nice comment, but the sentence was just present in the draft ;)''

'''Theory of constraints:'''
* Nice illustration. However, it will make it easier to read it, if you made it bigger.
''Thanks for the tips. I am thinking that in order to avoid making the article too long, some of the figures will remain of the same size. To better visualize them it possible to click on it.''
* Good use of a quote. You might want to consider mentioning the one you are quoting by name e.g. ‘As explained by Goldratt…’

'''Undesired effects of traditional approach:'''
* I like the mentioning of a ‘’Student Syndrome’’
* Again, good use of a figure, but it needs to be bigger
''Thanks again. See previous comment''

'''Suggestions for improvement:'''
* Remember the bibliography of references
* It might be an idea to have examples in the article. It would also be interesting in comparing Critical Path to CCPM.
''Due to space limitation, I choose to deeply analyze the CCPM and I add specific explanation step by step with an external example.''

I’m looking forward to reading your finished article. Good luck!

==Review3, s150794:==

'''Overall:'''
* My first impression of this article is that it is clear and straightforward, and looks good with clearly defined headings and sections, and there are descriptive pictures with explanation.
* You have a good academic language throughout the article.
* The article is a bit short, but I see that you have some more topics you want to write about. :)
'''Abstract:'''
* A good introduction to the term, but I'm not sure if you need the first sentence.
'' I forgot to remove it.''
'''Theory of constraints:'''
* I like that you start with a short introduction of this topic.
* It think it is a smart way to highlight the implementation how you use the 5 points from the first paragraph to the next, with an adequate explanation.

'''Undesired effects of traditional approach:'''
* Looks good so far. I look forward to the rest 
'''Some suggestions for improvement:'''
* There are some space between some sentences that may give the text less flow. Tips could be to make the first part of “Theory of construction” to make it to a coherent paragraph.

''I remove the spaces where unnecessary, but I did not get the second part of your sentence.''
* Some sentences that where a bit long, which made it a bit hard to follow. A tip is to either divide of rewrite the sentences. Two examples:
** “CCPM is an outgrowth of the Theory of Constraints(TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, “Critical Chain”[2] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.”
** “TOC is a systems-management philosophy, originally applied to production system, based on the principle that any system must have a constraint that limits its output.”
'' Thanks for noting this. It is an "italian problem" I am trying to avoid. I shortened many sentences''
* Some of the sentence had some grammar faults. I would looked a little on sentence structure.
'' Thanks. Hope I found all the grammar mistakes''
* In addition to CCPM you could include a paragraph on risk, since you have this in the title.

''It would be a really nice idea. Since risk is a quite wide topic, I am not sure if it would fit in my article.''

==Reviewer 2, s145170:==

'''Overall:'''
* The article clearly follows the “methods” structure, states the topic from the beginning and is compact and to the point.
* The topic is related to a project, program or portfolio topic
* Good and clear academic language
* Figures that correspond to the theory described and cover the main aspects of the topic
* The transition from the one headline to the other, as well as from sentence to sentence is logical

'''Abstract:'''
* Good description of the context that is directly related to the Method
* Important that you mention the Critical Path Method, which differs from the CCPM and you will compare them

'''Theory of constraints:'''
* Nice introduction to the topic by stating the relation between CCPM and TOC, which explains why you will analyze the second.
* Even if you describe the implementation of the “five focusing steps” to Project Management, you could refer to a real-world example, which would be combined with the analysis of the theory that you already did.
'' Thanks for the tip. I add a practical explanation regarding PM along the five focusing steps.''

'''Undesired effects of traditional approach:'''
* Important that you mention the undesired effects of traditional approaches, so that you strengthen the implementation of yours.
* I like the example with the student syndrome, as it helps understand what you describe in practice.

'''Suggestions of improvements'''
* As you cover the theory adequately (even if a part is missing, it is clear from the contents), a practical example is needed, maybe in the form of small practical implementations/examples like the “Student Syndrome”, in order to reinforce your theory description.
* You could also add your own opinion somewhere in the text. As it is difficult to include our own opinion in the description of a theory, you could evaluate the implementation of this theory in a small example, as I proposed one point before
''Thanks, I change a bit the structure and add pratical instruction both about five steps and about CCPM method itself.''
* Maybe you could avoid some voids between small sentences.
* If the figures are not yours, you may have to add a reference to avoid copyright problems
''I had the source in the description of the figures, not sure if it is the right place''

Very interesting topic with nice compact structure! I wish you good luck with the rest!

The Critical Chain Method

2015-09-28T22:27:20Z

F.labrini:

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability, and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:25:54Z

F.labrini:

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
The Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

The Critical Chain Method

2015-09-28T22:25:21Z

F.labrini:

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain method in Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Talk:The Critical Chain Method

2015-09-28T22:24:27Z

F.labrini: /* Review3, s150794: */

Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/different in the article.

==Reviewer 1, s112960==

'''Overall'''
* It looks to me as if you are following the ‘method’ structure
* Clearly the topic is related to a project, program or portfolio topic
* It’s written very clear and straightforward and is thus easy to read
* It’s an interesting topic.
* Even though the article is not done I like how it’s build up e.g. how ‘limitations’ of traditional approaches leads to the explanation of Critical Chain Method
* It’s good that you have many different references both books and online. This makes your article trustworthier.

'''Abstract'''
* Good explanation of why CCPM was made (response to projects resulted in larger duration etc.)
* If it is possible, it would be awesome if exactly how CCPM differs from traditional methodology is explained in one sentence so we already know in the beginning. You write it differs in how uncertainty is handled – but how exactly?
''Nice comment, but the sentence was just present in the draft ;)''

'''Theory of constraints:'''
* Nice illustration. However, it will make it easier to read it, if you made it bigger.
''Thanks for the tips. I am thinking that in order to avoid making the article too long, some of the figures will remain of the same size. To better visualize them it possible to click on it.''
* Good use of a quote. You might want to consider mentioning the one you are quoting by name e.g. ‘As explained by Goldratt…’

'''Undesired effects of traditional approach:'''
* I like the mentioning of a ‘’Student Syndrome’’
* Again, good use of a figure, but it needs to be bigger
''Thanks again. See previous comment''

'''Suggestions for improvement:'''
* Remember the bibliography of references
* It might be an idea to have examples in the article. It would also be interesting in comparing Critical Path to CCPM.
''Due to space limitation, I choose to deeply analyze the CCPM and I add specific explanation step by step with an external example.''

I’m looking forward to reading your finished article. Good luck!

==Review3, s150794:==

'''Overall:'''
* My first impression of this article is that it is clear and straightforward, and looks good with clearly defined headings and sections, and there are descriptive pictures with explanation.
* You have a good academic language throughout the article.
* The article is a bit short, but I see that you have some more topics you want to write about. :)
'''Abstract:'''
* A good introduction to the term, but I'm not sure if you need the first sentence.
'' I forgot to remove it.''
'''Theory of constraints:'''
* I like that you start with a short introduction of this topic.
* It think it is a smart way to highlight the implementation how you use the 5 points from the first paragraph to the next, with an adequate explanation.

'''Undesired effects of traditional approach:'''
* Looks good so far. I look forward to the rest 
'''Some suggestions for improvement:'''
* There are some space between some sentences that may give the text less flow. Tips could be to make the first part of “Theory of construction” to make it to a coherent paragraph.

''I remove the spaces where unnecessary, but I did not get the second part of your sentence.''
* Some sentences that where a bit long, which made it a bit hard to follow. A tip is to either divide of rewrite the sentences. Two examples:
** “CCPM is an outgrowth of the Theory of Constraints(TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, “Critical Chain”[2] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.”
** “TOC is a systems-management philosophy, originally applied to production system, based on the principle that any system must have a constraint that limits its output.”
'' Thanks for noting this. It is an "italian problem" I am trying to avoid. I shortened many sentences''
* Some of the sentence had some grammar faults. I would looked a little on sentence structure.
'' Thanks. Hope I found all the grammar mistakes''
* In addition to CCPM you could include a paragraph on risk, since you have this in the title.

''It would be a really nice idea. Since risk is a quite wide topic, I am not sure if it would fit in my article.''

==Reviewer 2, s145170:==

'''Overall:'''
* The article clearly follows the “methods” structure, states the topic from the beginning and is compact and to the point.
* The topic is related to a project, program or portfolio topic
* Good and clear academic language
* Figures that correspond to the theory described and cover the main aspects of the topic
* The transition from the one headline to the other, as well as from sentence to sentence is logical

'''Abstract:'''
* Good description of the context that is directly related to the Method
* Important that you mention the Critical Path Method, which differs from the CCPM and you will compare them

'''Theory of constraints:'''
* Nice introduction to the topic by stating the relation between CCPM and TOC, which explains why you will analyze the second.
* Even if you describe the implementation of the “five focusing steps” to Project Management, you could refer to a real-world example, which would be combined with the analysis of the theory that you already did.
'' Thanks for the tip. I add a practical explanation regarding PM along the five focusing steps.''

'''Undesired effects of traditional approach:'''
* Important that you mention the undesired effects of traditional approaches, so that you strengthen the implementation of yours.
* I like the example with the student syndrome, as it helps understand what you describe in practice.

'''Suggestions of improvements'''
* As you cover the theory adequately (even if a part is missing, it is clear from the contents), a practical example is needed, maybe in the form of small practical implementations/examples like the “Student Syndrome”, in order to reinforce your theory description.
* You could also add your own opinion somewhere in the text. As it is difficult to include our own opinion in the description of a theory, you could evaluate the implementation of this theory in a small example, as I proposed one point before
''Thanks, I change a bit the structure and add pratical instruction both about five steps and about CCPM method itself.''
* Maybe you could avoid some voids between small sentences.
* If the figures are not yours, you may have to add a reference to avoid copyright problems
''I had the source in the description of the figures, not sure if it is the right place''

Very interesting topic with nice compact structure! I wish you good luck with the rest!

Talk:The Critical Chain Method

2015-09-28T22:24:08Z

F.labrini: /* Reviewer 1, s112960 */

Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/different in the article.

==Reviewer 1, s112960==

'''Overall'''
* It looks to me as if you are following the ‘method’ structure
* Clearly the topic is related to a project, program or portfolio topic
* It’s written very clear and straightforward and is thus easy to read
* It’s an interesting topic.
* Even though the article is not done I like how it’s build up e.g. how ‘limitations’ of traditional approaches leads to the explanation of Critical Chain Method
* It’s good that you have many different references both books and online. This makes your article trustworthier.

'''Abstract'''
* Good explanation of why CCPM was made (response to projects resulted in larger duration etc.)
* If it is possible, it would be awesome if exactly how CCPM differs from traditional methodology is explained in one sentence so we already know in the beginning. You write it differs in how uncertainty is handled – but how exactly?
''Nice comment, but the sentence was just present in the draft ;)''

'''Theory of constraints:'''
* Nice illustration. However, it will make it easier to read it, if you made it bigger.
''Thanks for the tips. I am thinking that in order to avoid making the article too long, some of the figures will remain of the same size. To better visualize them it possible to click on it.''
* Good use of a quote. You might want to consider mentioning the one you are quoting by name e.g. ‘As explained by Goldratt…’

'''Undesired effects of traditional approach:'''
* I like the mentioning of a ‘’Student Syndrome’’
* Again, good use of a figure, but it needs to be bigger
''Thanks again. See previous comment''

'''Suggestions for improvement:'''
* Remember the bibliography of references
* It might be an idea to have examples in the article. It would also be interesting in comparing Critical Path to CCPM.
''Due to space limitation, I choose to deeply analyze the CCPM and I add specific explanation step by step with an external example.''

I’m looking forward to reading your finished article. Good luck!

==Review3, s150794:==

'''Overall:'''
* My first impression of this article is that it is clear and straightforward, and looks good with clearly defined headings and sections, and there are descriptive pictures with explanation.
* You have a good academic language throughout the article.
* The article is a bit short, but I see that you have some more topics you want to write about. :)
'''Abstract:'''
* A good introduction to the term, but I'm not sure if you need the first sentence.
'' I forgot to remove it.''
'''Theory of constraints:'''
* I like that you start with a short introduction of this topic.
* It think it is a smart way to highlight the implementation how you use the 5 points from the first paragraph to the next, with an adequate explanation.
'''Undesired effects of traditional approach:'''
* Looks good so far. I look forward to the rest 
'''Some suggestions for improvement:'''
* There are some space between some sentences that may give the text less flow. Tips could be to make the first part of “Theory of construction” to make it to a coherent paragraph.

''I remove the spaces where unnecessary, but I did not get the second part of your sentence.''
* Some sentences that where a bit long, which made it a bit hard to follow. A tip is to either divide of rewrite the sentences. Two examples:
** “CCPM is an outgrowth of the Theory of Constraints(TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, “Critical Chain”[2] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.”
** “TOC is a systems-management philosophy, originally applied to production system, based on the principle that any system must have a constraint that limits its output.”
'' Thanks for noting this. It is an "italian problem" I am trying to avoid. I shortened many sentences''
* Some of the sentence had some grammar faults. I would looked a little on sentence structure.
'' Thanks. Hope I found all the grammar mistakes''
* In addition to CCPM you could include a paragraph on risk, since you have this in the title.

''It would be a really nice idea. Since risk is a quite wide topic, I am not sure if it would fit in my article.''

==Reviewer 2, s145170:==

'''Overall:'''
* The article clearly follows the “methods” structure, states the topic from the beginning and is compact and to the point.
* The topic is related to a project, program or portfolio topic
* Good and clear academic language
* Figures that correspond to the theory described and cover the main aspects of the topic
* The transition from the one headline to the other, as well as from sentence to sentence is logical

'''Abstract:'''
* Good description of the context that is directly related to the Method
* Important that you mention the Critical Path Method, which differs from the CCPM and you will compare them

'''Theory of constraints:'''
* Nice introduction to the topic by stating the relation between CCPM and TOC, which explains why you will analyze the second.
* Even if you describe the implementation of the “five focusing steps” to Project Management, you could refer to a real-world example, which would be combined with the analysis of the theory that you already did.
'' Thanks for the tip. I add a practical explanation regarding PM along the five focusing steps.''

'''Undesired effects of traditional approach:'''
* Important that you mention the undesired effects of traditional approaches, so that you strengthen the implementation of yours.
* I like the example with the student syndrome, as it helps understand what you describe in practice.

'''Suggestions of improvements'''
* As you cover the theory adequately (even if a part is missing, it is clear from the contents), a practical example is needed, maybe in the form of small practical implementations/examples like the “Student Syndrome”, in order to reinforce your theory description.
* You could also add your own opinion somewhere in the text. As it is difficult to include our own opinion in the description of a theory, you could evaluate the implementation of this theory in a small example, as I proposed one point before
''Thanks, I change a bit the structure and add pratical instruction both about five steps and about CCPM method itself.''
* Maybe you could avoid some voids between small sentences.
* If the figures are not yours, you may have to add a reference to avoid copyright problems
''I had the source in the description of the figures, not sure if it is the right place''

Very interesting topic with nice compact structure! I wish you good luck with the rest!

Talk:The Critical Chain Method

2015-09-28T22:23:46Z

F.labrini: /* Reviewer 1, s112960 */

Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/different in the article.

==Reviewer 1, s112960==

'''Overall'''
* It looks to me as if you are following the ‘method’ structure
* Clearly the topic is related to a project, program or portfolio topic
* It’s written very clear and straightforward and is thus easy to read
* It’s an interesting topic.
* Even though the article is not done I like how it’s build up e.g. how ‘limitations’ of traditional approaches leads to the explanation of Critical Chain Method
* It’s good that you have many different references both books and online. This makes your article trustworthier.

'''Abstract'''
* Good explanation of why CCPM was made (response to projects resulted in larger duration etc.)
* If it is possible, it would be awesome if exactly how CCPM differs from traditional methodology is explained in one sentence so we already know in the beginning. You write it differs in how uncertainty is handled – but how exactly?
''Nice comment, but the sentence was just present in the draft ;)''

'''Theory of constraints:'''
* Nice illustration. However, it will make it easier to read it, if you made it bigger.
''Thanks for the tips. I am thinking that in order to avoid making the article too long, some of the figures will remain of the same size. To better visualize them it possible to click on it.''
* Good use of a quote. You might want to consider mentioning the one you are quoting by name e.g. ‘As explained by Goldratt…’

'''Undesired effects of traditional approach:'''
* I like the mentioning of a ‘’Student Syndrome’’
* Again, good use of a figure, but it needs to be bigger
''Thanks again. See previous comment''
'''Suggestions for improvement:'''
* Remember the bibliography of references
* It might be an idea to have examples in the article. It would also be interesting in comparing Critical Path to CCPM.
''Due to space limitation, I choose to deeply analyze the CCPM and I add specific explanation step by step with an external example.''

I’m looking forward to reading your finished article. Good luck!

==Review3, s150794:==

'''Overall:'''
* My first impression of this article is that it is clear and straightforward, and looks good with clearly defined headings and sections, and there are descriptive pictures with explanation.
* You have a good academic language throughout the article.
* The article is a bit short, but I see that you have some more topics you want to write about. :)
'''Abstract:'''
* A good introduction to the term, but I'm not sure if you need the first sentence.
'' I forgot to remove it.''
'''Theory of constraints:'''
* I like that you start with a short introduction of this topic.
* It think it is a smart way to highlight the implementation how you use the 5 points from the first paragraph to the next, with an adequate explanation.
'''Undesired effects of traditional approach:'''
* Looks good so far. I look forward to the rest 
'''Some suggestions for improvement:'''
* There are some space between some sentences that may give the text less flow. Tips could be to make the first part of “Theory of construction” to make it to a coherent paragraph.

''I remove the spaces where unnecessary, but I did not get the second part of your sentence.''
* Some sentences that where a bit long, which made it a bit hard to follow. A tip is to either divide of rewrite the sentences. Two examples:
** “CCPM is an outgrowth of the Theory of Constraints(TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, “Critical Chain”[2] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.”
** “TOC is a systems-management philosophy, originally applied to production system, based on the principle that any system must have a constraint that limits its output.”
'' Thanks for noting this. It is an "italian problem" I am trying to avoid. I shortened many sentences''
* Some of the sentence had some grammar faults. I would looked a little on sentence structure.
'' Thanks. Hope I found all the grammar mistakes''
* In addition to CCPM you could include a paragraph on risk, since you have this in the title.

''It would be a really nice idea. Since risk is a quite wide topic, I am not sure if it would fit in my article.''

==Reviewer 2, s145170:==

'''Overall:'''
* The article clearly follows the “methods” structure, states the topic from the beginning and is compact and to the point.
* The topic is related to a project, program or portfolio topic
* Good and clear academic language
* Figures that correspond to the theory described and cover the main aspects of the topic
* The transition from the one headline to the other, as well as from sentence to sentence is logical

'''Abstract:'''
* Good description of the context that is directly related to the Method
* Important that you mention the Critical Path Method, which differs from the CCPM and you will compare them

'''Theory of constraints:'''
* Nice introduction to the topic by stating the relation between CCPM and TOC, which explains why you will analyze the second.
* Even if you describe the implementation of the “five focusing steps” to Project Management, you could refer to a real-world example, which would be combined with the analysis of the theory that you already did.
'' Thanks for the tip. I add a practical explanation regarding PM along the five focusing steps.''

'''Undesired effects of traditional approach:'''
* Important that you mention the undesired effects of traditional approaches, so that you strengthen the implementation of yours.
* I like the example with the student syndrome, as it helps understand what you describe in practice.

'''Suggestions of improvements'''
* As you cover the theory adequately (even if a part is missing, it is clear from the contents), a practical example is needed, maybe in the form of small practical implementations/examples like the “Student Syndrome”, in order to reinforce your theory description.
* You could also add your own opinion somewhere in the text. As it is difficult to include our own opinion in the description of a theory, you could evaluate the implementation of this theory in a small example, as I proposed one point before
''Thanks, I change a bit the structure and add pratical instruction both about five steps and about CCPM method itself.''
* Maybe you could avoid some voids between small sentences.
* If the figures are not yours, you may have to add a reference to avoid copyright problems
''I had the source in the description of the figures, not sure if it is the right place''

Very interesting topic with nice compact structure! I wish you good luck with the rest!

Talk:The Critical Chain Method

2015-09-28T22:23:29Z

F.labrini: Created page with "Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/diff..."

Mette: Hello, I like your idea and I like if you would add the topic you have written within the brakets. As you write yourself it would be nice to add something specific/different in the article.

==Reviewer 1, s112960==

'''Overall'''
* It looks to me as if you are following the ‘method’ structure
* Clearly the topic is related to a project, program or portfolio topic
* It’s written very clear and straightforward and is thus easy to read
* It’s an interesting topic.
* Even though the article is not done I like how it’s build up e.g. how ‘limitations’ of traditional approaches leads to the explanation of Critical Chain Method
* It’s good that you have many different references both books and online. This makes your article trustworthier.

'''Abstract'''
* Good explanation of why CCPM was made (response to projects resulted in larger duration etc.)
* If it is possible, it would be awesome if exactly how CCPM differs from traditional methodology is explained in one sentence so we already know in the beginning. You write it differs in how uncertainty is handled – but how exactly?
''Nice comment, but the sentence was just present in the draft ;)''
'''Theory of constraints:'''
* Nice illustration. However, it will make it easier to read it, if you made it bigger.
''Thanks for the tips. I am thinking that in order to avoid making the article too long, some of the figures will remain of the same size. To better visualize them it possible to click on it.''
* Good use of a quote. You might want to consider mentioning the one you are quoting by name e.g. ‘As explained by Goldratt…’

'''Undesired effects of traditional approach:'''
* I like the mentioning of a ‘’Student Syndrome’’
* Again, good use of a figure, but it needs to be bigger
''Thanks again. See previous comment''
'''Suggestions for improvement:'''
* Remember the bibliography of references
* It might be an idea to have examples in the article. It would also be interesting in comparing Critical Path to CCPM.
''Due to space limitation, I choose to deeply analyze the CCPM and I add specific explanation step by step with an external example.''

I’m looking forward to reading your finished article. Good luck!

==Review3, s150794:==

'''Overall:'''
* My first impression of this article is that it is clear and straightforward, and looks good with clearly defined headings and sections, and there are descriptive pictures with explanation.
* You have a good academic language throughout the article.
* The article is a bit short, but I see that you have some more topics you want to write about. :)
'''Abstract:'''
* A good introduction to the term, but I'm not sure if you need the first sentence.
'' I forgot to remove it.''
'''Theory of constraints:'''
* I like that you start with a short introduction of this topic.
* It think it is a smart way to highlight the implementation how you use the 5 points from the first paragraph to the next, with an adequate explanation.
'''Undesired effects of traditional approach:'''
* Looks good so far. I look forward to the rest 
'''Some suggestions for improvement:'''
* There are some space between some sentences that may give the text less flow. Tips could be to make the first part of “Theory of construction” to make it to a coherent paragraph.

''I remove the spaces where unnecessary, but I did not get the second part of your sentence.''
* Some sentences that where a bit long, which made it a bit hard to follow. A tip is to either divide of rewrite the sentences. Two examples:
** “CCPM is an outgrowth of the Theory of Constraints(TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, “Critical Chain”[2] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.”
** “TOC is a systems-management philosophy, originally applied to production system, based on the principle that any system must have a constraint that limits its output.”
'' Thanks for noting this. It is an "italian problem" I am trying to avoid. I shortened many sentences''
* Some of the sentence had some grammar faults. I would looked a little on sentence structure.
'' Thanks. Hope I found all the grammar mistakes''
* In addition to CCPM you could include a paragraph on risk, since you have this in the title.

''It would be a really nice idea. Since risk is a quite wide topic, I am not sure if it would fit in my article.''

==Reviewer 2, s145170:==

'''Overall:'''
* The article clearly follows the “methods” structure, states the topic from the beginning and is compact and to the point.
* The topic is related to a project, program or portfolio topic
* Good and clear academic language
* Figures that correspond to the theory described and cover the main aspects of the topic
* The transition from the one headline to the other, as well as from sentence to sentence is logical

'''Abstract:'''
* Good description of the context that is directly related to the Method
* Important that you mention the Critical Path Method, which differs from the CCPM and you will compare them

'''Theory of constraints:'''
* Nice introduction to the topic by stating the relation between CCPM and TOC, which explains why you will analyze the second.
* Even if you describe the implementation of the “five focusing steps” to Project Management, you could refer to a real-world example, which would be combined with the analysis of the theory that you already did.
'' Thanks for the tip. I add a practical explanation regarding PM along the five focusing steps.''

'''Undesired effects of traditional approach:'''
* Important that you mention the undesired effects of traditional approaches, so that you strengthen the implementation of yours.
* I like the example with the student syndrome, as it helps understand what you describe in practice.

'''Suggestions of improvements'''
* As you cover the theory adequately (even if a part is missing, it is clear from the contents), a practical example is needed, maybe in the form of small practical implementations/examples like the “Student Syndrome”, in order to reinforce your theory description.
* You could also add your own opinion somewhere in the text. As it is difficult to include our own opinion in the description of a theory, you could evaluate the implementation of this theory in a small example, as I proposed one point before
''Thanks, I change a bit the structure and add pratical instruction both about five steps and about CCPM method itself.''
* Maybe you could avoid some voids between small sentences.
* If the figures are not yours, you may have to add a reference to avoid copyright problems
''I had the source in the description of the figures, not sure if it is the right place''

Very interesting topic with nice compact structure! I wish you good luck with the rest!

The Critical Chain Method

2015-09-28T22:22:38Z

F.labrini: Created page with "Category:Project ManagementCategory:SchedulingCategory:UncertaintyCategory:Theory of Constraints Critical Chain Project Management(CCPM) is a method to plan, e..."

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|400| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Articles Fall Term 2015

2015-09-28T22:22:05Z

F.labrini: /* Overview of 2015 Wiki Articles */

Please complete this table with your name, user name and the title of your article.

To create more lines in the table click '''Edit''' and use the following code to create more lines in the table and replace the example text with your own information:

<pre style="white-space: pre-wrap;
white-space: -moz-pre-wrap;
white-space: -pre-wrap;
white-space: -o-pre-wrap;
word-wrap: break-word;">
|Group Number
|Last Name
|First Name
|Username
|Link to Article
|-

Create a direct link by making square brackets around the title [[Title]] (Case sensitive)

The straight lines ( | ) create columns and the straight line with a dash ( |- ) creates a new row in the table.
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</pre>

=Overview of 2015 Wiki Articles=

{| class="wikitable sortable"
|+Fall 2015 Wiki Articles
|-
!Group Number
!Last Name
!First Name
!User Name
!Link to article
|-
|Group 1
|Nguyen
|Michael
|Michaelnguyendtu
|[[Leading an offshore team]]
|-
|Group 12
|Gkatzalas
|Nikolaos
|s141569
|[[The Gantt chart and the usage nowadays]]
|-
|Group 6
|Lymperis
|Konstantinos
|s142330
|[[Risk Management in Oil and Gas Industry]]
|-
|Group 3
|Filis
|Charalampos
|Ch.filis
|[[Project Risk Management and Project Risk Management Processes]]
|-
|Group 11
|Larsen
|Leonora
|s112910
|[[Gantt Charts as a Tool for Project Management]]
|-
|Group 10
|Sala Vilar
|Lluís Ròmul
|s141586
|[[Portfolio Management in a Startup]]
|-
|Group 14
|Pitsavas
|Konstantinos
|Konspits
|[[Modularisation: A modern process for project management]]
|-
|Group 2
|Kampianakis
|Andreas
|s150912
|[[Financial Portfolio Optimization Methods]]'''
|-
|Group 13
|Penzes
|Balint
|s141943
|[[Product development and portfolio management processes at LEGO]]
|-
|Group 6
|Hozmache
|Mihaela
|s146898
|[[PRINCE2 - For successful Project Management]]'''
|-
|Group 14
|Le Corre
|Damien
|Damien
|[[Game theory in project management]]'''
|-
|Group 14
|Bertrand
|Fabien
|150477
|[[Multi project management]]'''
|-
|Group 9
|Cassel
|Sara
|Sarac
|[[The benefits of systems engineering]]'''
|-
|Group 13
|Sergi
|Gibaja Musachs
|S141926
|[[Manage Extreme Projects with Rapid Methodology]]'''
|-
|Group 14
|Poza
|María
|s150793
|[[Integrated Cost and Schedule Control]]'''
|-
|Group 6
|Kulikova
|Nataliia
|s140767
|[[SCRUM Method]]
|-
|Group 4
|Pekala
|Adam
|Adam.pekala
|[[Critical Path Method in Construction Industry]]
|-
|Group 10
|Garnotel
|Gaëtan
|gaetangarnotel
|[[V-Model]]
|-
|Group 4
|Ghanizada
|Naweed
|S103745
|[[PRINCE2, A Project Management Methodology]]
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|Group 1
|Jacobsen
|Martin
|MistaJacob
|[[Mindfulness and Cognitive Biases in Project Management]]
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|Group 6
|Ferraresi
|Fabrizio
|S150905
|[[Projects in Controlled Environments, a process-based approach for project management]]
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|Group 5
|Tanghus
|Bjarke
|S113815
|[[Location Based Scheduling]]
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|Højgaard Hindhede
|Daniel
|S143352
|[[Construction modularization from a lean perspective]]
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|Gayot
|Charles-Henri
|s141074
|[[Responsibility Assignment Matrix (RACI Matrix)]]
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|Thorp Sørensen
|Anders
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|[[The Gantt Chart]]
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|Makris
|Dimitrios
|Dimak
|[[Benchmarking in Project Management]]
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|Greiling
|Lea
|Lea
|[[Lean in Project Management]]
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|Latorre Duque
|Ana
|Ana
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|Almanzi
|Stefano
|S141530
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|Montagner
|Giacomo
|S150821
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|Ruiz Muñoz
|Gustavo Adolfo
|S121408
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|Group 3
|Kalmus
|Thomas
|S141938
| [[Program evaluation and review technique (PERT) ]]
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|Group 12
|Gudmundsson
|Arnar Gauti
|S141543
|[[Management of risk]]
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|Group 5
|Jacobsen
|Ian Thobias
|S113735
|[[Story Points Estimation]]
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|Boesgaard
|Katrine
|KB1991
|[[Gantt Chart]]
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|Sorth-Olsen
|Rasmus
|Sorth90
|[[Lean as a project management tool]]
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|Group 13
|Salling
|Stephanie
|StephSalling
|[[E. Pihl & Søn A/S from a management perspective]]
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|Ruina
|Jessica Linda
|Jejenji
|[[Scheduling techniques in Project Management]]
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|Gjerstrup
|Jacob
|s113440
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|Lynge
|Jane
|s997303
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|Palmerini
|Alessandro
|alex161
|[[Effective Communication in Project Management]]
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|Tvedt
|Ida Marie
|IMT
|[[Risk Profile in Turnkey Projects]]
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|Søndenaa
|Mathilde Hanssen
|s150621
|[[Critical chain project management (CCPM)]]
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|Helassalo
|Antti
|s141506
|[[Development phase of idea to project]]
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|Thorning-Schmidt
|Nanna
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|[[Earned Value Management]]
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|Edvinas
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|Marie-Louise
|DI2009
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|Fernando
|s131882
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|S145170
|[[Olympic Games London 2012: When the client strives for innovation (The London model)]]'''
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|Moe
|Elizabeth Lindhard
|113129
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|Lessis
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|Klibo Buur
|Christian
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|[[Project Execution Model (PEM)]]
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|Vilar Bustos
|Alberto
|s142581
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|Group 1
|Trap Wiegandt
|Sissel
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|Christensen
|Britt Marie Lekven
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|[[Lean in building and construction industry]]
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|Group 5
|Vestergaard Andersen
|Andreas
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|[[Management of Project Change ]]
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|Ann-Elise
|Gustavsen
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|[[Stakeholder Analysis and Matrices ]]
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|Daniel
|DanielKrogh
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|-
|Group 7
|Fabio
|Labrini
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|Viig
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|s102935
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|Hansen
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|Otiv
|Peter
|s145166
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|Bianka Zsuzsanna
|Biankajuh
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|Rodrigues
|Rafael
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|Camilla
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|Ilektra
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|}

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:16:11Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:15:57Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG|300| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:15:32Z

F.labrini: /* 3. Subordinate everything else to the above decision */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG|300px| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:15:11Z

F.labrini: /* Planning */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|400px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:14:56Z

F.labrini: /* Control */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|250px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
[[File:Fever_chart.PNG|400px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.

In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:14:18Z

F.labrini: /* Critical Chain Method */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG|250px| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG|250px| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:10:24Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T22:08:23Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the [https://en.wikipedia.org/wiki/Diversification_%28finance%29 Diversifcation]applied to Project Management by protecting the project from common cause uncertainty of the individual activities. It aggregates the safety times of task in the same path placing a buffers at the end of the path. Buffers appear as activities in the project plan, but have no work assigned to them.
The statistical law allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name=’’CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:59:51Z

F.labrini: /* Suggested resources */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''

This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''

This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.

- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''

Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.

- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''

This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method

- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''

The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.

- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''

The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:59:12Z

F.labrini: /* Suggested resources */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''
This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- '''Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute'''
This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.
- '''Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House'''
Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.
- '''Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34'''
This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method
- '''Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference'''
The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.
- '''Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5'''
The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:58:34Z

F.labrini: /* Suggested resources */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''
This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- Larry P. Leach, (1997), Critical Chain Project Management Improves Project Performance, Advanced Projects Institute
This article describes how the Theory of Constraints is applied to project management, the main features of the CCPM. In the final part resumes all the advantages of the method and list a series of company which successful implemented the CCPM.
- Lawrence P. Leach, 2005, Critical chain Project Management, 2ed, Artech House
Critical Chain Project Management is a book which deeply explains the CCPM. It starts from the TOC, then explains CCPM applied to both single and multi project environments. In addition, it has a chapter about risk management.
- Raz T., Barnes R., Dvir D., (2003), A critical look at critical chain project management, Project Management Journal, vol. 34
This article analyzes the principles of CCPM with a review of its key elements, followed by a critical analysis of the method. Then the conclusion provide an honest evaluation of the method
- Kirpes C, Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference
The article evaluate the cause that help CCPM to be able to provide successful result in terms of project on time, with the established budget and the proper quality. Furthermore, after having presented advantages and company which adopted the method, it questions why the method is not widely adopted.
- Marris P., (2011), La chaîne critique pour réduire le time to market et accroître la productivité, STP PHARMA PRATIQUES vol.21 N°5
The article analyze the CCPM and its main characteristics, with special regards to Drug Development. Moreover, it presents the results of the application of CCPM in many pharmaceuticals companies.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:57:20Z

F.labrini: /* Suggested resources */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''
This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- {{#ev:youtube|https://www.youtube.com/watch?v=mpc_FdAt75A|200|left|'''Critical Chain Project''' by Project Management Videos.}}

Theoretical, short, and clear explanation of Critical Chain Project Management.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:50:04Z

F.labrini: /* Suggested resources */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- '''Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies'''
This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- {{#ev:https://www.youtube.com/watch?v=mpc_FdAt75A|200|center|''Theoretical, short, and clear explanation of Critical Chain Project'' Management.}}
Theoretical, short, and clear explanation of Critical Chain Project Management.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:47:50Z

F.labrini:

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= Suggested resources =

- Woeppel M., (2005), ‘’Projects in Less Time:: A Synopsis of Critical Chain’’, Pinnacle Strategies
This book helps to understand the Goldratt’s book “Critical Chain”. It follows chapter by chapter the book “Critical Chain”, analyzing it. Then, at the end of each chapter there is a section where the author interprets the content of the chapter and gives his point of view.

- {{#ev:youtubehttps://www.youtube.com/watch?v=mpc_FdAt75A|200|center|Theoretical, short, and clear explanation of Critical Chain Project Management.}}
Theoretical, short, and clear explanation of Critical Chain Project Management.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:40:48Z

F.labrini: /* CCPM as a solution */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.

=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:40:25Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb||'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
[[File:Multitasking.PNG|220px|thumb| |'''Figure 4:''' Multitasking's effect.]]Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.

Figure 5 helps to better understand this process.
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:38:22Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb|left|'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

[[File:Multitasking.PNG|220px|thumb|left |'''Figure 4:''' Multitasking's effect.]]
5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task.
Figure 5 helps to better understand this process.
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:35:47Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG|200px|thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG|200px|thumb|left|'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG|250px|thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

[[File:Multitasking.PNG|220px|thumb|left |'''Figure 4:''' Multitasking's effect.]]
5. '''Multitasking: increase the completion time'''
An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:28:58Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb|left|'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

[[File:Multitasking.PNG| thumb|left |'''Figure 4:''' Multitasking's effect.]]
5. '''Multitasking: increase the completion time'''
An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:28:14Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb|left|'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''
[[File:Multitasking.PNG| thumb|left |'''Figure 4:''' Multitasking's effect.]]
An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:27:42Z

F.labrini: /* Undesired effects of traditional approaches */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb|left|'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity[[File:Multitasking.PNG| thumb|left |'''Figure 4:''' Multitasking's effect.]] paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:26:35Z

F.labrini: /* Practical Example */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb| |'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity[[File:Multitasking.PNG| thumb| |'''Figure 4:''' Multitasking's effect.]] paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|left|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:26:20Z

F.labrini: /* Practical Example */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb| |'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity[[File:Multitasking.PNG| thumb| |'''Figure 4:''' Multitasking's effect.]] paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|right|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:25:51Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb| |'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity[[File:Multitasking.PNG| thumb| |'''Figure 4:''' Multitasking's effect.]] paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The sizing of the FB follows the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|center|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:25:38Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb| |'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity[[File:Multitasking.PNG| thumb| |'''Figure 4:''' Multitasking's effect.]] paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.

- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.

- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.

The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The size of the FB follow the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|center|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>

Critical Chain Project Management to cope with uncertainty

2015-09-28T21:25:11Z

F.labrini: /* 2. Decide how to exploit the project’s constraints */

[[Category:Project Management]][[Category:Scheduling]][[Category:Uncertainty]][[Category:Theory of Constraints]]
Critical Chain Project Management(CCPM) is a method to plan, execute, manage, and control both single and multi projects, which emphasizes the effects of resource allocation and activity duration uncertainty.
It has demonstrated over the past 10 years its ability to significantly reduce the duration of projects, to ensure that projects are completed on time, and to increase resource productivity.<ref name="FR">Marris P., (2011), ''La chaîne critique pour réduire le time to market et accroître la productivité'', STP PHARMA PRATIQUES vol.21 N°5</ref>.
CCPM is an outgrowth of the [[Theory of Constraints]](TOC) and was introduced in 1997 in Eliyahu M. Goldratt’s book, [http://www.goldratt.co.uk/resources/critical_chain “Critical Chain”] in response to many projects resulted in larger duration, increased cost, and less derivable than expected.

The Critical Chain method mainly differs from the traditional methodologies* on the attention it places on the resource availability and flexibility of starting time rather than task order and strict scheduling.

In this paper, with traditional methods it is referred to [[The Critical Path Method (CPM)]] and [[Program evaluation and review technique (PERT)]]

== Undesired effects of traditional approaches ==
This section describes the factors that contribute to delay the completion of a project and the effects on human behavior produced by some of them.

1. '''Excessive Activity Duration Estimates'''

People, when are requested to estimate an activity duration, attempt to make commitments that they could meet with a high level of certainty<ref> Steyn H., (2000), ''An investigation into the fundamentals of critical chain project scheduling'', International Journal of Project Management 19, pp. 363-369</ref>. In addition, managers selectively remember the[[File:The_student_syndrome.PNG| thumb| |'''Figure 1:''' The student syndrome leads.]]instances where activity duration estimates were exceeded, and therefore wants to add contingency of his own.
The combination of both actions lead to a final duration estimation with a probability of completion of 80% to 95% on or less than the activity duration estimate<ref name="CCPMPer"></ref>.

2. '''Performance overrun estimates'''

Even tough, as previously stated, the estimates are quite padded, performance exceeds estimates.

Many people have a tendency to wait until activities get really urgent before they work on them<ref name="CCPMPer"></ref>.
This tendency is better known as ''Student Syndrome'': a person will only start to apply themselves to an assignment at the last possible moment before its deadline<ref>https://en.wikipedia.org/wiki/Student_syndrome</ref>.
By acting in this way, people tend to waste their contingency before they start the activity, forcing them to perform most of the work in the
later portion of the scheduled activity time. Then, if problems occur, there is no time to recover<ref name="CCPMPer"></ref>.

[[File:Parkinson_Law.PNG| thumb| |'''Figure 2:''' The Parkinson's Law.]]
3. '''Failure to pass on early completion '''

Analysis of almost any project’s results reveals that people report very few activity as completed early<ref name="CCPMPer"></ref>. Since the estimates are usually around 80-90% of probability of completion, the results should shown an higher number of activities completed early.
On the one hand, the reason behind those result is the relationship between the level of performance and the established goal. If applied to Project Management environment, it leads to the ''Parkinson’s Law''. The latter described by Parkinson(1957) as “work expands so as to fill the time available for its completion”. A loose deadline(i.e. lowering the goal), leads, thus, to a decline of the worker’s performance and to a delay of the activity<ref>Gutierrez G. J., Kouvelis P.,(1991), ''Parkinson’s law and its implication for Project Management'', Management Science Vol. 37, No. 8</ref>.

On the other hand, even if activities completed early, people fail to report. People have little or no reward on early completion. In addition, if the activity is completed early, the worker gets more to do, and the next time he will have to replicate the same performance. In other words, reducing future estimates for the same task.
[[File:Merging_paths.PNG| thumb| |'''Figure 3:''' Activity path merging.]]

4. '''Activity path merging creates delay'''

Usually projects have multiple activity paths which must merge into the critical path before the completion of the project. Merging activity[[File:Multitasking.PNG| thumb| |'''Figure 4:''' Multitasking's effect.]] paths means that all of the feeding paths(activity path that merges or feeds the critical one) are required to start the successor activity.
Therefore, the successor activity can not start until the latest of the merging activities completes<ref name="CCPMPer"></ref>. In a scenario as the one represented by figure 4, even if a feeding path is completed in advance, the positive variation is wasted.

5. '''Multitasking: increase the completion time'''

An additional reason which contributes to make tasks longer is multitasking. When an individual is working on more than one activity/project simultaneously, for instance three, each task completion time would result in a three time longer duration. This occurs since the individual spends one third of its time in each activity, causing an extension in the project duration as the successor activity has now to wait three times the original duration of the single task . Figure 5 helps to better understand
=== CCPM as a solution ===

The reason for the development of Critical Chain is the existence of chronic problems that existing methods, approaches and even expensive software have not been able to remove <ref name="Rand">Rand GK, (2000), ''Critical chain: the theory of constraints applied to
project management'', International Journal of Project Management, vol.18, pp.173±7</ref>.

In project management, variation is related to the intrinsic uncertainty of task durations. The latter will vary according to two reasons<ref> Deming, W. Edwards, (1989), ''Out of the Crisis'', MIT Press</ref>:

- Common cause variation: cause inherent with the system. It is a variation in duration that predictably occur since its origin is part of the system within project are performed.

- Special cause variation: variation in parts of the task process, which makes those parts of the process unpredictable. The effects are unpredictable.

In addition, W. Edwards Deming [1989] notes: “I should estimate that in my experience most troubles and most possibilities for improvement add up to propositions something like this: 94% belong to the system, 6% special.” Meaning that the majority of the problems and the biggest room for improvement are related to common cause variation.

Traditional project scheduling, such as PERT or Monte Carlo analysis, estimates the impact of common cause variation, and accounts for it adding safety time at task level (each project task). Furthermore, they do not take into account the impact of negative resource behavior, described in the previous section.
While these methods propose a way to estimate uncertainty, they do not pose an effective systematic method to manage it. CCPM accounts for common cause variation as an essential element of the project management system<ref name="CCPMPer"></ref>. CPPM addresses common cause variation through the use of an aggressive schedule with shortened task duration estimates, and the use of buffer placed in strategic positions, aggregating the protection by removing safety from each tasks.

== Theory of Constraints ==

Since CCPM applies TOC’s concepts to project management, it is useful to understand the reasoning behind the theory.[[File:Five_steps.PNG| thumb| |'''Figure 5:''' The five focusing steps represent the TOC approach to ongoing improvement.]]
TOC is a systems-management philosophy, originally applied to production system. It is based on the principle that any system must have a constraint that limits its output. If there were no constraints, system output would either rise indefinitely or would fall to zero.
Therefore, a constraint( or bottleneck) limits any system with a nonzero output<ref name="CCPM2"> Lawrence P. Leach, 2005, ''Critical chain Project Management'', 2ed, Artech House, ISBN 1-58053-903-3</ref>.
A system’s constraint may be physical (e.g. materials, machines, people, demand level) or managerial<ref> Rahman S., (1998), ''Theory of Constraints: A review of the philosophy and its applications'', International Journal of Operations and Production Management. 18(4), pp. 336-355</ref> which hinders the system to achieve better performance.

At first, Goldratt states “before we can deal with the improvement of any section of a system, we must first define the system’s global goal; and the measurements that will enable us to judge the impact of any subsystem
and any local decision, on this global goal”<ref name="CCPM2"></ref>.
Then in order to improve the system’s performance, the limiting constraint must be found and improvement efforts should be placed on elevating the capacity of that constraint.
Goldratt defined the “five focusing steps” as an continuous improvement process<ref> Graham K. Rand, (2000), ''Critical chain: the theory of constraints applied to project management'', International Journal of Project Management 18, pp. 173±177</ref> :

1. Identify the system's constraint(s)

2. Decide how to exploit the system's constraint(s)

3. Subordinate everything else to the above decision

4. Elevate the system's constraint(s)

5. If, in the previous steps, the constraint has been broken, go back to step 1, and do not allow inertia to cause a system's constraint

The five steps process permits to identify the most detrimental constraint. When the latter is solved, the next constraint needs to be identified and addressed. Therefore, it is a continuous improvement process.

== TOC to Project Management ==

Concepts and principles of CCPM are obtained by applying the TOC improvement process to
Project Management.

In order to apply the five steps, it is necessary at first to define the goal of a project. The primary goal of a project is considered to be the promised project due date.

===1. '''Identify project’s constraint'''===
According to the TOC, the part of the system that constrains the objective is the ''Critical Chain'', which is defined as “the longest chain of precedence and resource dependent tasks that determines the overall duration of a project”. Defining the constraint of a project in
terms of the schedule derives from the impact that schedule has on project cost and project scope. The three conditions are dependent<ref name="CCPMPer">Larry P. Leach, (1997), ''Critical Chain Project Management Improves Project Performance'', Advanced Projects Institute</ref>.
Since the Critical Path does not account resource allocation while determining a schedule, if resources would be infinite then Critical Path
and Critical Chain would be identical.

===2. '''Decide how to exploit the project’s constraints'''===
[[File:Estimate_distribution.PNG| thumb| |'''Figure6:''' Typical probability distribution for a task with uncertain distribution.]]
This step can be translated in focusing on the activities of the Critical Chain to ensure efficient performance and no delays. In order to
achieve this, the critical factors leading to delays should be identified.

''' Exploit task duration estimates '''

As stated previously, usually task duration estimates are determined with a probability of completion around 90%.
In order to minimize the negative resource behavior and to avoid wasting the contingency allocated to the single tasks, CCPM entails to remove the safety time from the task estimates and shorten them down to 50% of probability.
Contingency is the difference between a 50% probable estimate and a 90% probable estimate<ref name="CCPM2"></ref>.
Critical Chain method requires that the schedule be built with only the time to do the work without any safety. This is the time we expect the work to take if allowed to focus a full sustainable level of effort on it and if there are no significant problems<ref name=”frank”> Patrick S.F., (1999), ''Critical chain scheduling and buffer management - getting out from between Parkinson’s rock and Murphy’s hard place'', PM Network 13, pp. 57-62</ref>

''' Exploit statistical law of aggregation '''

CCPM exploits the statistical law of aggregation by protecting the project from common cause uncertainty
of the individual activities in an activity path with buffers at the end of the path. Buffers appear as
activities in the project plan, but have no work assigned to them.
The statistical law of aggregation of variances allows to protect a chain of tasks to the same level of probability with much less total contingency time than you can protect each individual task<ref name="CCPMPer"></ref>.
By aggregating the safety times, the whole estimated duration of a chain of tasks(project) is heavily reduced.

There are three types of buffers:

- Project Buffer(PB): a time buffer placed at the end of the critical chain, between the last task and the completion date. The Project Buffer acts as a protection in response to any delays, which consequently consume some of the buffer. The Project Buffer size is usually half of the size of the safety time taken out from the critical chain.
- Feeding Buffer(FB): a time buffer placed at the end of each paths feeding the critical chain, between the last task on a feeding path and the critical chain. The Feeding Buffers avoid the critical chain to be delayed if any delays occur in the feeding paths.
- Resource Buffer: buffer that act as warning signals to assure the availability of resources. It is placed alongside the critical chain.
The size of the PB is equal to the sum of the safety times removed from the task duration estimates of the tasks along the critical chain. The size of the FB follow the same procedure accounting for the tasks preceding the buffer.

=== 3. '''Subordinate everything else to the above decision'''===
[[File:Feeding_bufferOK.PNG| thumb| |'''Figure7:'''Feeding buffer absorbs delay from feeding paths.]]
Applied to project, it means that all the non-critical activities must not affect or delay any activity on the critical tasks.

''' Subordinate Merging Paths '''

To protect the critical chain from potential delays by subordinating critical chain feeding paths, CCPM places an aggregate FB on each path that feeds the critical chain<ref name="CCPM2"></ref>. In addition, the FB acts as means to measure and control the feeding paths.
Figure 7 shows how the FB preserves the critical chain from potential delays in the non-critical paths.
CCPM also involves the late start of all activities in the feeding paths. This is done since the FB provides protection to the whole project from the non-critical path. The advantages of doing so are the following: reduce the impact of changes on work already performed, delay the project cash outlay, and give the project a chance to focus by starting with fewer simultaneous activity chains<ref name="CCPMPer"></ref>.

=== 4. '''Elevate the project’s constraints''' ===
When no more improvement can be obtained, this step suggests to invest in additional resources,
or increase the capacity of resources that can benefit most the critical chain performance.

''' Elevate task performance '''

The failure to pass on early completion and the propensity to multitask are two factors which contribute to extend the project duration.
CPPM tackles those tendencies above-mentioned by only providing exact starting dates of the activity chains and the exact end date of the PB. Approximated start times and estimated activity durations are provided too as reference values.
“Relay Runner” mentality is included in which resources begin work as soon as assigned, work without interruption until done, and announce when they finish immediately when task criteria is fully met<ref> Kirpes C., (.2014), ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management'', Proceedings of the 2014 Industrial and Systems Engineering Research Conference Y. Guan and H. Liao, eds.</ref>.
The elimination of task due dates, the encouragement of focusing on the task at hand, and the enforcement of a “Relay Runner” behavior contribute to increase the capacity of the resources. The latter is obtained by passing on early completion and avoiding multitasking.

=== 5. '''If, as a result of the previous steps, the constraint has alleviated, return to Step 1''' ===

== Critical Chain Method ==
===Planning===
[[File:Project_buffer.PNG| thumb| |'''Figure 8:''' Transfer of the task level safety to the project level.]]
The first step in order to apply CCPM consists in defining an initial schedule taking in consideration task duration estimates and dependencies. In other words, the initial schedule could be also defined as the “infinite capacity schedule” since the resource availability is not considered yet.
Thereafter, the schedule must be adjusted by positioning the tasks according to the capacity and availability of the resources. Because at least some of the resources have limited availability, the resulting schedule is likely to be longer than the schedule obtained with the basic Critical Path Method, as critical activities are delayed while waiting for the resources they require<ref name="Clook"> Raz T., Barnes R., Dvir D., (2003), ''A critical look at critical chain project management'', Project Management Journal, vol. 34</ref>. Then, the longest sequence of activities needed to complete the project taking into account both the interdependencies and resource capacity is defined as the Critical Chain.
Next step involves shortening duration estimates, removing the safety margin from each critical task and pooling them together at the PB. The latter is place at the end of the project, and represented as a task.
Then, the same process of grouping safety margins is applied to the non-critical paths. The safety margins of each non-critical path are grouped into a FB placed where the path merges into the critical chain path.

===Execution===

Once the new project schedule is created, consisting of tasks with reduced durations and different types of buffer, the project manager needs to execute the project plan.
During this phase, the resource working on the critical chain activities must work continuously on a single activity at time. They are not allowed to work on tasks in parallel in order to avoid multitasking, for the reason explained above.
To further enforce the resource behaviors a “Relay Runner” mentality must be followed by the workforce. To enhance this mentality, the management should reward the completion of individual activities early.
In case of early completion of a task, work on the successor activity must begin rapidly. In the opposite scenario, there is no reason for immediate concern, as the buffer will absorb the delay <ref name="Clook"></ref>.

===Control===

In CCPM, managing and tracking of project performance is based on buffer consumption: as the project progresses, the manager constantly check how much of the buffer is consumed to protect both the critical chain(FB) and the project due date(PB) from disruption.
Buffers are supposed to act as transducers that provide vital operational measurement and a proactive warning mechanism. If activity variation consumes a buffer by a certain amount, a warning is raised<ref> Herroelen W., Leus R., (2001), ''On the mertis and pitfalls of critical chain scheduling'', Journal of Operations Management, vol. 19, pp. 559-577</ref>. Then, if the consumption rate is quite high so that the whole buffer could be consumed before the end of the project, corrective actions must be taken.
[[File:Fever_chart.PNG| thumb| |'''Figure 9:''' Fever chart measuring project performance.]]
In order to have facilitate the monitoring and evaluation of the buffer consumption, buffers are usually divided in three third, respectively represented as Green zone, Yellow zone, Red zone.
The level of buffer consumption gives management visible signals:

- Green zone: no action

- Yellow zone: assess the problem and prepare for action

- Red zone: corrective action must be implemented

Through this mechanism, buffer management provides a unique anticipatory project-management tool
with clear decision criteria<ref name="CCPM2"></ref>.

However, the buffer consumption has to be combined with the project completion to reflect a clear and accurate project status. The tricolored chart used to visualize the project status, where the buffer consumption is plotted against the project completion(both expressed as a percentage), is called Fever Chart.

===Practical Example===

{{#ev:youtube|https://www.youtube.com/watch?v=t803JLLoi38|200|center|Critical Chain Practical example by Ron Torre}}
The video shows an application of the steps described in the paper.
It shows an example of an application of CCPM along with [https://en.wikipedia.org/wiki/Microsoft_Project Microsoft Project ]. It starts by explaining a simple example with three tasks, followed by a more complex one.
In the video he uses the word critical path to define the path containing the critical activity, while in this paper it is referred as critical chain.

== Benefits ==

Although CCPM is relatively new concept, it has proved to achieve successful results in terms of expected benefits.
All projects that have properly applied the methodology have completed the project under the original time estimate, fulfilled the original scope, and came in near or under the estimated budget. Project durations normally reduce by at least 50% in the first pass, and several companies have taken the early successes to cause further substantial reductions in project duration <ref name="CCPMPer"></ref>.

The following companies have implemented the CCPM with positive results: Israeli Aircraft Industry, US Air Force, Johnson & Johnson, Procter & Gamble, Ely Lilly Boeing, ITT, BAE System, and Harris<ref name="CCPMPer"></ref><ref name="IOWA">Kirpes C, ''Evaluating the Use of Scheduling Techniques: Critical Chain Project Management’’, Proceedings of the 2014 Industrial and Systems Engineering Research Conference</ref><ref name="FR"></ref>. However, the result obtained are not only attributable to the CCPM but also to the quality of the actor involved, and to the weak starting situation of company.

The following are some of the advantages or differences from traditional scheduling methods which mainly caracterize CCPM.
Those include: accounting the uncertainty in duration by explicating the buffer and sharing its size with all the people involved in a project(workers, management, stakeholders), considering the resource availability and scheduling consequently, providing a clear overview of the project status and visibility of threats while allowing for intervention.

== Limitations ==

Despite all of these project benefits, Critical Chain is still not widely adopted in industry<ref name="IOWA"></ref>. A reason can be that CCPM has been criticized about the newness of its principles as well as the certainty of them.
It has been argued that experienced project managers have known the principles behind CCPM for decades, and CCPM’s uniqueness is in the terminology rather than in its substance<ref name="Clook"></ref>.

Firstly, one of the main critique regards the CCPM indication of reducing the task duration estimates by a specific percentage, “which seems arbitrary”<ref name=”MIT”> S.C. Cook, (1998) ''Applying critical chain to improve the management of uncertainty in project'', MS thesis, Massachusetts Institute of Technology, USA</ref>.
The method, by suggesting this fixed reduction, assumes that all resources overestimate task duration by the same amount. The latter it is not true since the amount overestimated depends on the individual and it is based on personality, job experience, nature of the task, workload, or other reasons<ref name="Clook"></ref>.
Furthermore, resources who know that their estimates will be reduced could be motivated to increase or double the estimates. By doing so, they still have their safety margins even after the reduction.

Secondly, while an advantage of CCPM is creating a schedule that considers resource availability, actually the mathematical problem of creating a precedence and resource dependent schedule is a well known NP-hard problem<ref> Wuliang P, Minli J., (2009), ''A Revised Critical Chain Method and Optimization Model'', Applied Mechanics and Materials vols. 16-19, pp. 426-430</ref>. It means that there are no efficient algorithms for finding optimal schedule for large project. Consequently, heuristic rules, which generate solution that are hoped to be close to optimum, are used<ref name="Clook"></ref>.

Thirdly, CCPM involves the elimination of multitasking as a way to improve performance. However, a study of 64 high technology firms conducted by McCollum and Sherman (1991)<ref> McCollum, J.K., & Sherman, J.D., (1991), ''The effects of matrix organization size and number of project assignments on performance'', IEEE Transactions on Engineering Management vol.38, pp.75-78</ref> found that there is a relationship between the number of projects assigned to R&D personnel and key performance of the company. They found that assignment of two projects is the optimal number, and until three projects no problems occurred.
This study alone cannot confirm that the optimal number, in regards to performance, of project assigned to one individual is two, since it was just conducted in R&D environment. Further studies are necessary.

Finally, another criticism is about the adoption of CCPM. A company that want to adopt the CCPM has to incur in two big expenses: provide training to various levels of the organization, and purchase the proper softwares. In order to be efficient, CCPM requires training to change organizational culture about: giving up ownership of the task duration and relying on the schedule buffer to absorb deviation, replacing the concept of due date with estimated completion date range(represented by Feeding and Project Buffers), and avoiding multitasking<ref name="Clook"></ref>.

= References =
<References/>