Critical Chain Method CCM in Project Management C

The Critical Path Method helped project managers develop and manage the schedule in the past. 

This method makes the life of a project manager easy. They can plan activities with less effort, and this was a good communication tool.

However, there were many issues with the schedules developed by critical path methods. These schedules were not realistic, and projects started getting delayed, which caused crossing their cost baselines.

Many times these delays led to project termination, and this was hurting companies financially.

Issues with the Critical Path Method

The critical path method has a few inherent drawbacks:

  • Unlimited Resources
  • Float Misuse 
  • Activity Completion Gain/Loss
  • Student Syndrome

Unlimited Resources

The critical path model is optimistic. It assumes that the project has all resources at all times and whenever needed. 

However, this is not always possible. Many times, this assumption leads to a delay in projects and additional costs. No organization can provide unlimited resources for any project. In fact, resource constraints are a fact.

Misuse of Float 

Misuse of Float or slack was common in the Critical Path Method.

According to Parkinson’s Law, “work expands to fill the time available for its completion.” Team members were misusing the slack, causing delays in the schedule.

If any activity has float, the team member will use all of it and then complete the task. They were considering the float as a part of the activity duration estimate.

Activity Completion Gain/Loss

If you complete an activity early, the time gain was useless because the next has to wait until its early start date. This may happen because the resources allocated to the next activity may not be available at the moment.

However, the opposite is not true. If any activity is delayed, it will affect the next. The next activity will be delayed and so the project is too. Delays accumulate in the critical path method, but gain does not.

Student Syndrome

Student Syndrome infects the critical path. Team members do not start the task until the last moment.

Therefore, the activity has no float and any delay in the activity will affect the project schedule. 

So, the critical path-based schedule has many issues.

There was a need to develop a pragmatic approach to developing a realistic schedule that can help project managers complete on time with minimal obstruction. 

Hence, the Critical Chain Method (CCM) came into existence. Mr. Eliyahu M. Goldratt developed this method in 1997 as an improvement over the Critical Path Method.

Many experts call the critical chain method as critical chain project management 

What is the Critical Chain Method (CCM)?

Before discussing the Critical Chain Method, let’s understand the critical chain.

The critical chain is “the longest path in the network diagram considering activity interdependence and resource constraints.”

critical chain network diagram

(Path “Start->C->D->E->F->End” is the critical chain.)

Now we come to the Critical Chain Method.

The Critical Chain Method is an updated form of the Critical Path Method. Here, you consider resource availability while developing the project schedule.

In the Critical Chain Method, you use a buffer instead of float. These buffers eliminate the concept of float or slack.

You can consider the critical path as a particular case of the critical chain when the project has unlimited resources.

Critical chain management has three buffers. These buffers are:

  1. Project Buffer
  2. Feeding Buffer
  3. Resource Buffer

Project Buffer

This buffer is placed between the last task and the project completion date as a non-activity buffer, and it acts as a contingency for the critical chain activities. Any delay on the critical chain will eat this buffer and the project completion date will not change. 

If any activity finishes early, the gain will be added to this buffer.

Usually, the duration is 50% of the contingency that you have removed from each task. This helps to move uncertainty from the tasks to the project buffer, improve efficiency and reduce the schedule duration.

Please note that although the critical chain starts at the beginning, it ends before the start of the project buffer. It does not end at the close of the project. This duration will include any time duration borrowed from the project buffer or exclude the duration added to the buffer.

Feeding Buffers

This buffer is added to the non-critical chain so that any delay on the non-critical chain does not affect the critical chain. Feeding buffers are inserted between the last task on a non-critical chain and the critical chain.

Feeding buffers and the project buffer are calculated the same way. The duration of these buffers is based on some fraction of the safety removed from the tasks on non-critical chains.

Resource Buffer

Resource buffers are kept alongside the critical chain to ensure that they are available when required. These buffers can be human resources or equipment.

Please note that since the critical chain considers resource constraints, its duration will be longer than the critical path. However, you can compensate for this by removing contingencies from the activities. 

The resources used in the critical chain are known as critical resources.

Differences Between Buffer and Float

Many aspirants confuse buffer and float. They find these terms similar; however, they are not.

  • Float or slack is a critical path phenomenon, while buffer belongs to the critical chain.
  • A float is the difference between the duration of the critical and the non-critical paths. A buffer is based on contingencies.
  • A float is zero on a critical path, while a buffer is not zero on a critical chain or any other chain.
  • A float is the same for all activities on a non-critical path, any activity can consume it partially or fully, and others can utilize the balance. There is no further analysis.
  • Any activity can also borrow a buffer if the activity is delayed. The project manager analyzes the remaining buffer to find the status of the project.
  • Buffers can be divided into three categories: project buffer, feeding buffer and resource buffer. A float is either total float or free float.

How to Create the Critical Chain Network Diagram

To create a critical chain network diagram, first, create a critical path network diagram. 

Then you have to follow these three additional steps to develop a critical chain network diagram:

  1. Remove all contingencies from activities. Replace your estimate with an optimistic estimate if you have used a PERT (Program Evaluation and Review Technique) estimate to build the schedule.
  2. Align the activities with late finish dates and remove resource constraints. Give priority to critical chain activities while assigning resources.
  3. Add feeding buffers to non-critical chains so that their durations are equal to the critical chain. Add project buffer to the end of the critical chain, but before the project end date. 

You can see that the Critical Chain Method is a modified form of the Critical Path Method.

A Real-World Example of Critical Chain Method

Suppose you get a project to construct a building. You develop a schedule based on the Critical Path Method and start the work.

However, during the execution of this project, you come to know that:

  • There is a shortage of cement, or
  • Equipment from your project is assigned to some other projects, or
  • Management has pulled one team member for some urgent work.

What will happen now?

Of course, this will cause a delay in your project.

So, where was the problem?

Did the critical path not identify the resources required by your project?

No, the critical path identified the resources for your activities.

So, where was the problem? What went wrong?

The problem was with resource allocation. Although the critical path identified the resources, it did not account for the limited availability of resources. The project schedule was developed with the assumption that all resources would be available whenever they were needed. Unfortunately, this could not happen, and the schedule was delayed.

Therefore, to resolve these issues, you apply resource constraints to your critical path and it is now a critical chain network diagram and it is more realistic. 

Now you can complete your project with more confidence.

Let us revisit some key features of critical chain management before this blog post ends:

  • It is a deterministic model.
  • It avoids mismanagement of slack or float.
  • It optimizes the utilization of resources.
  • Projects based on the critical chain method are completed 10% to 30% faster than those based on the critical path method.
  • It is a more practical approach.
  • It improves productivity.

Summary

Critical Chain Method is a more practical approach to developing the project schedule. In it, the availability of resources is taken into consideration while drawing the network diagram. Here you use buffers instead of float. The Critical Chain Method is one of the most important developments in project management made recently. This method addresses many shortcomings of the Critical Path Method, provides a realistic schedule, encourages team members to perform efficiently, and improves productivity.

Are you involved with schedule development? Please share your experiences with the critical chain technique.

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Speak Your Mind

  • Hello Fahad,

    Then you have to follow these three additional steps to develop a critical chain network diagram:

    1)Remove all contingencies from activities. Replace your estimate with an optimistic estimate if you have used a PERT (Program Evaluation and Review Technique) estimate to build the schedule.
    2)Align the activities with late finish dates and remove resource constraints. Give priority to critical chain activities while assigning resources.
    3) Add feeding buffers to non-critical chains so that their durations are equal to the critical chain. Add project buffer to the end of the critical chain, but before the project end date.
    You can see that the Critical Chain Method is a modified form of the Critical Path Method.

    Can you please help with the diagrammatic explanation for the above. I didn’t get this thing.

  • Good day Fahad, First time coming across this artical and sounds very interesting. I personally think this could be a good implementation on the African construction market personally knowing that a lot of development is happening. Kindly advise of how long the training takes and perhaps we can run a training course here in South Africa. Or perhaps how can i get hold of you to discuss the logistics.

  • Really this is a very good and informative post. i am first time going through such useful information. Thanks you so much for providing such a helpful information…

  • Hello, i’d like to ask about Critical chain scheduling.
    Can I use 3 point pert estimation for the initial schedule?
    If I can, since RSEM method use standard deviation, should i use standard deviation from PERT estimation for Critical Chain?
    If you can give me some reference, i will gladly appreciate your help. Thank you

    • Hello Timothy,

      These days we are using critical chain method to develop a schedule. Critical path is basic concept. Yes you can use three point estimate for developing a network diagram.

      What is RSEM?

  • I agree with all of your assumptions about using CCPM except one. I believe one of the assumptions of critical chain is that estimated activity durations are not “deterministic” ?

    • Critical chain method is not mentioned by name in the 6th edition of the PMBOK Guide, but it is a schedule network diagram technique.

  • Hi Fahad,

    Been following your articles and write up – quite resourceful and really enlightening.

    My confusing part is this: it is mentioned that project buffer is located before the project end, assuming that all buffers within critical chain are removed and accumulated in project buffer.

    My question is, how do we calculate the project buffer and the whole project duration?. My understanding is that :
    1. project buffer is calculated based on 50% from the activitie(s) performed before – the last activity.
    2. project buffer duration is part of the project duration

    Thanks for your kind help

  • Hi Fahad,
    I have question related to the below statement
    ” Also, if there is any gain from the early finish of any activity, this gain will be added to this buffer as well”
    Question:
    Suppose task A has 3 days duration and B has 2 days duration.B is dependent on A. so task responsible of B will be available on 4th day of project schedule.If I complete task A on 2nd itself ,then what is the gain Iam getting here? Task B responsible is not available on 3rd day.What is the purpose of adding this gain in project buffer?

  • Dear Mr Fahad

    Nice explaination once again, would like to know , how the uncertainity of resource availablity( like in your examle cement, manpower , machine ) will affect the values in buffers.

  • I have read explanations on the Critical Chain Method numerous times from all sorts of book and references, none of them gave me the understanding you have given me. Thank you very much.

  • What a nice article on CCM, I really appreciate easy and simple language used to explain this concept which even PMBOK cant explain in this detail. Thanks a lot for putting this article.

  • Dear Sir,
    Very nice Elaboration if CCM. I am looking for Article on CPM. Please let me know I I can get it

  • Hi Fahad,

    Your explanations for each topics is really simple and it has appropriate explanations to understand the concepts easily. I understood CPM, Float and few more topics were really awesome.

    Thanks for your efforts, in sharing this to us. I am grateful to you, for developing my knowledge. 🙂

    Regards,
    Mohan

  • Hello Fahad,

    If we consider optimist estimates for CCM (50% approach) to eliminating padding and instead of that adding a buffer at the end without work assigned, the total planned cost will be less than total planned cost in case of estimation in 95% approach (or activities padded). So in case of using the buffer, when we create baseline and we compare total planned cost with current cost, we will CV negative. What can we do in case of give the buffer to project instead of padding tasks, to avoid from CV negative?

    Thanks in advance.
    Ali.

    • I assigned same resources to the buffer with the amount of time/cost associated with the padding removed from the actual tasks, This way the total project cost remained the same with the potential project savings being the buffer if not used. If the project goes as planned, no adverse risks arised, then the padding time (buffer) was not used and the project remained on schedule from start to end. Otherwise if risks did turn to issues and the schedule was affected, then I still had the buffer time to keep me on track for the end date and I had the buffer of costs associated so the project did not go over budget since the buffer was a planned expense and time from the start.

  • Fahed
    thank you for your reply , but i believe you misunderstand my question , i understand critical path meaning , but still buffering in CCM not clear !!
    kindly can you explain it again if you don’t mind

    Thanks

  • Fahed
    what i understood that CCM methodology is depend on cutting the contingency in the estimation time for each activity ans sharing it together and put it as project buffer , while in critical path you added special buffer to compensate the shortage of resources which also depend the cut from the contingency , and feeding resources which you put in non critical path also the me methodology

    it means we just cut from each activity and paste it as a total time for project , am i right ?? if Yes it means that i just cut all contingency and put it as a buffer !!

  • Thanks Fahad for a nice explanation of the concept! Here I would like to take the opportunity to ask you one thing. For Float, it is hidden in the schedule and people (especially client) cannot dig it out because we are sharing PDF version or hard copy of schedule with them (as strategy). But for Buffers, it seems difficult to hide and being a PM I have to handle the stakeholders strategically. So, can you please make some advice in this case.
    Regards,
    Shafi

  • Excellent explanation in a very simplified way..to add to this readers can refer to the book “Critical chain” – by Dr.Eliyahu Goldratt where this concept is explained in a story format…happy reading !!

  • Hello;

    I am trying to implement critical chain idea in ms project, I know I need to cut my tasks duration, add buffers, and create schedule on late finish dates. Do you have any example or materials that I can refer to as steps example.

    Thank you

    • Sorry Hussein, I don’t have any material that you are looking for. However, I suggest you try finding some good reference books on the amazon. I believe it will help you implement critical chain concepts on your project.

  • Dear Fahd,
    I will be thankful if you show these buffers on NWD and explain by aid of example like you did for critical path in one of your blogs.
    How is FB calculated….is it . LS of successor in critical chain minus LF of predecessor (in non critical chains last activity which joins critical path)…. same hows RB calculated..
    Thanks in advance n stay blessed

    • Basic concept will always be same. You will review the change request, get it approved (if needed) and then implement it.

  • Dear Fahad,

    I became a bit confused now about the critical chain, resource leveling and crashing, would you explain this for me,

    Resource leveling : is move resources.

    Crashing : is adding resources.

    Critical chain : is reallocating resource as a result of limited resources.

    Many Thanks
    aboturke

    • In resource leveling you move source from one activity to another activity. On the other hand in crashing you add additional resources to the activity which increased the cost to the project.

      In critical chain you use the resource leveling.

          • Hi Fahad,

            I have been reading all your updates with regards to PMP, however this time I am not satisfied with your CCM blog, kindly explain how to calculate the buffer with simple example.

            Thanks
            Rohit
            9999084558

          • I believe that you need to demonstrate the procedure to create CCM, using the available software, either P6 or Ms.P, my understanding that creating a normal CPM using P6- as example- with Resource leveling will lead to the same CCM, Am I right???

            Many thanks

          • I believe that you’ve missed the practical part, I mean how to implement the CCM in any of the recognized planning softwares.
            My understanding to apply that method, you can go directly to P6-as example- and start establish your time plan loaded with ideal resources, then set the restricted resources’ limits and re leveling to redistribute the limited resources. Then , at that time you’ll have your schedule resource buffer and project buffer. Am I right….?

            Many thanks.

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