Critical Path and Critical Chain Method: PMP Topics to Understand

Quick quiz! Which of the following is NOT a tool or technique in the Develop Schedule Process: CPM, CCM, or CCR?

CCR, of course! They’re a classic rock band. (You’re welcome for the background music.)

Let’s start with the basics to make sure you know your acronyms:

  • CPM – Critical Path Method
  • CCM – Critical Chain Method

PMBOK defines CPM as “a method used to estimate the minimum project duration and determine the amount of scheduling flexibility on the logical network paths within the schedule model”, while “CCM is a schedule method that allows the project team to place buffers on any project schedule path to account for limited resources and project uncertainties” (176).

CCM builds on the methodology of CPM by more specifically defining a project buffer.

For the purposes of the PMP exam, think of CCM as a more sophisticated version of CPM. In order to really understand how CCM is determined, let’s first look at CPM.

Critical Path Method (CPM)

Critical Path Method is a good name, because CPM is all about the critical path. The critical path is the sequence of activities that cannot be delayed if a project is to finish on time. CPM uses a diagram to show the progress of a project. CPM is addressed starting on page 176 of the PMBOK Guide.

Here’s the basic structure of an activity node in the CPM:

critical chain method pmp

Early Start and Finish the earliest times an activity can start and finish, while Late Start and Finish are the latest.

  • Total float is the amount of float for a sequence of activities. Total float on the critical path, as you might imagine, is zero.
  • Free float is float of a particular activity. Free float of an activity on the critical path is zero.

Now let’s imagine a project where you are building a tomato garden. Your activities are as follows:

  • A – Select a plot of land, which can happen immediately after the start and takes 2 days.
  • B – Build the garden, which comes after A and takes 7 days.
  • C – Purchase the seeds & tomato cages, which can happen immediately after the start and takes one day.
  • D – Plant the seeds, which takes place after B & C and takes one day.

Here’s the project in a network diagram:

critical path pmp

Here’s how we determine the critical path. This time, we’re only looking at the Early Start, Duration, and Early Finish lines at the top. Take a close look at activity D, because it’s the most complicated one in this forward pass..

critical chain method pmp

Your critical path, then is A-B-D. Notice also that to determine D’s start, you have to choose the later of B’s or C’s finish. Now let’s do the backward pass.

critical path pmp

Now, for determining the late finish of activity C, you need to choose the latest time that it can end without impacting the critical path. That’s the day before D starts, so C can finish on Day 9. The float of Activity C is its Late Finish minus its early finish, which is 8.

I’ll make a few more important terminology introductions here about resource smoothing vs. leveling, with the caveat that this article is only a brief introduction.

  • Resource smoothing is a first effort at making sure the schedule stays on time. The technique moves around resources based on free and total float to ensure that the critical path is unaffected.
  • Resource leveling is a heavier-handed approach that actually changes the critical path. It is usually used when

Float is addressed on page 177, and resource optimization is covered beginning on page 178 of the PMBOK guide. These topics are often tested on the PMBOK in conjunction with CPM diagrams, so get to know them well.

Critical Chain Method

CCM exists because there is a fatal flaw with CPM. CPM doesn’t allow for any slack in the system — if you are just one day late on an activity on the critical path, your project will be delayed. For that reason, there is a strong incentive to misrepresent the length of activities on the critical path. Think about it: if you know that you have absolutely no wiggle room on a particular activity on a project, wouldn’t would overestimate that activity duration a bit to ensure that you finish on time? I would!

CCM adjusts for this problem in CPM by building in buffers. In fact, if the only fact your remember about the CCM is that it involves buffers, you’re about 90% of the way towards understanding what you need to know for the PMP exam.

There are two types of buffers:

  • A feeding bufferis not on the critical chain.
  • A project buffer is at the end of the critical chain.

For CCM, you should know the characteristics of buffers and why buffers are a reasonable way to address the shortcomings of CPM. You won’t have to recreate the CCM chart on the exam like you will have to recreate the CPM one.

You Aren’t Finished Yet!

Remember that this article is a brief introduction to CPM and CCM, and it is written in way that hopefully makes the concepts a little more relatable. Project Time Management, however, is a crucial part of the PMP exam. For some areas, a short article describing the tool or technique is sufficient, but I recommend that you do lots of practice problems to ensure you fully understand this area.

You will get multiple questions in this area on your exam, so make sure you can accurately calculate all parts of a CPM diagram as well as accurately describe a CCM diagram. Project Time Management is not an area where you want to skim in the PMBOK Guide.

How are you doing with CPM and CCM? Comment below, and let’s discuss!

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