Critical Path and Critical Chain Method: PMP Topics to Understand


Critical Path Method

One of the most important schedule network analysis techniques today is the Critical Path Method also known as CPM in Project Management. The Critical Path is the longest sequence of activities that are required to be completed in order for completing the entire project. These activities might delay a project if not completed on time as this aggregation of activities comprise the entire project. It is basically used by the Project Managers to schedule and estimate the total duration of the project. The Critical Path Method is very important because:

  • It enables to estimate the project completion time
  • Identify the sub-tasks individually to understand the resource constraints if any or even if there are potential project risks
  • Prioritize each task / identify critical tasks in order to create feasible schedules to achieve realistic timelines of completion
  • Monitor/control the progress of the project / understand schedule variance if any
  • Use crash duration or fast-tracking

Project Managers use the Critical Path Method algorithm to derive the minimum time taken to complete each activity with minimum slack. So, what is a slack?  It is the time delay in a particular activity that doesn’t contribute to the delay of the project completion time. Slack can also be called a Float.

So, a Critical Path Method is usually represented by a network diagram that comprises a sequence of activities required to be completed for the entire project completion as shown below. Once these activities are sequenced and the paths are defined, the duration to complete each and every activity is mentioned from which we can derive the path that will take us the minimum time to complete the entire project.

Some important points to keep in mind while going for a Critical Path Method

The presence of more than one Critical Path

If there is more than one critical path within the same time frame then all the critical paths should be taken into consideration to meet the project completion deadline

Several Critical Paths pose high project risks

This essentially means that the successful completion of the project now depends on those several critical paths that comprise their unique set of activities which makes the process unpredictable due to the complexity of their functions, therefore, posing risks on the completion.

Critical Path can change over time

Let's say initially you assumed the near-critical path was ten days and the actual critical path was eleven days. Now, for some reason, an activity gets completed a couple of days early. So, it then becomes important to keep updating these changes in the project plan.


Critical Chain Method

The critical Chain Method is another schedule network analysis method that primarily takes resource limitations, task dependencies and buffers into account to prepare a project schedule. A critical path does not take these variables into consideration but a critical chain does which makes it a more reliable and suitable design to arrive at the deadline. So, we can say that a critical chain method is just an updated version of a critical path method. All the high-risk activities are first scheduled in the earlier stages of the project which allows these activities to get completed early and also gives buffers to any unexpected problems if they arise.

What is a buffer? The buffers eliminate the concept of slack in the path by becoming a safety net for certain activities with a lack of resources that eliminates/reduces delay of the project and protects the promise of the project completion date.

What is the main objective of the Critical Chain Method? It is to reduce/eliminate any unnecessary slack or uncertainties in a project.

In the below representation we see six primary steps in a critical chain method.

There are three types of buffers in the critical chain method

  1. Project buffer

    This is nothing but the total buffer of the project that we get from adding up the individual buffers from each task.

  2. Feeding buffer

    A feeding buffer is usually added in the non-critical path so that this buffer doesn’t cause any delay along the critical path.

  3. Resource buffer

    This buffer typically being human resources or equipment is fed into the activities along the critical path. The critical chain’s duration will always be longer than the critical path’s duration because of the resource constraints.


Example of a Critical Chain Network Diagram

Let us assume that there is a construction project that needs to be completed within a certain timeframe. So, we design a corresponding critical path in order for us to complete the project. We begin the work. However, along the way, some limitations come up. For example,

  1. Shortage of bricks
  2. The products/equipment of this particular project is being diverted to another project
  3. An important team member gets assigned to another project

These are typical examples of unexpected resource limitations which we had not taken into consideration while designing the critical path for the project. But now that these appear our schedule gets delayed due to these unforeseen circumstances and so we are to include these variables in our critical path which then becomes our critical chain method analysis. This network design contributes to a more realistic timeline of completion of the project. 

Hence, a critical chain method is always a more complete network design and hence is preferable to a critical path because, unlike a critical path, here we take into account resource limitations which can show up uninvited at any given time/stage of the project which then allows us to be prepared for sudden uncertainties without causing a delay in the overall project completion.

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