5th Edition PMBOK® Guide–Chapter 6: Critical Chain Method


One of the tools and techniques used in the process 6.6 Develop Schedule is the critical chain method. Unfortunately, it can be easily confused with the similar-sounding critical path method, which was covered in the last two posts.

The purpose of this post is to explain briefly what the critical chain method is, how it is distinguished from the critical path method, and how the addition of buffers used in the critical chain method differ from merely padding the estimates, a practice frowned upon by PMI.

1. The Critical Chain Method

First of all, why do you need the critical chain method in the first place? In the critical path method, you are given the durations of each individual activity as definite amounts: 7 weeks, for example, to complete activity A as opposed to 5 weeks to complete activity B.

However, when you start to use three-point estimates, you realize that the duration estimates for activities are better expressed in terms of a range of durations: 5-9 weeks, for example, to complete activity A as opposed to 3-7 weeks to complete activity B.

Because the activities themselves have a range of durations, once you figure out the critical path based on, say, the most likely durations of activities, you will have a float for activities on the non-critical path which now will also have a range. There is therefore some uncertainty about the durations of the individual activities and the total duration of a non-critical path. Buffers are added to account for this uncertainty. So if an activity has a float of between 1 and 3, you would place a buffer from 1 and 3 weeks to make sure that the non-critical path feeds into the critical path at the correct time.

There are two kinds of buffers: feeding buffers for each of the non-critical paths, and then a project buffer at the end of the critical path, which the project manager can use to make sure the project finishes on time.

2. The Critical Chain Method vs. the Critical Path Method

The critical path method deals with definite durations, whereas the critical chain method is used to add buffers that account for uncertain durations (i.e., those that can be expressed as a range). It adds a level of flexibility to the critical path method, if you want to put it in those terms.

3. Buffer vs. Padding

How does adding a buffer in the critical chain method differ from simply padding an estimate? Padding an estimate means adding an arbitrary, across-the-board percentage to one’s estimates. Padding is therefore an extremely blunt instrument for making sure your project is done within the schedule. It is wasteful, in that it may add to estimates of certain activities that don’t require any such “padding.” And the arbitrariness means that there really is no basis other than guesswork in how much you add to the estimates.

With a buffer, you are adding extra built-in time to the non-critical paths of the project a) at the precise point where they are needed and b) in the precise amount which is needed. So it is a scalpel as opposed to the blunt instrument of padding, and is therefore approved by PMI, whereas padding is frowned upon by PMI.

The next post deals with the next tool and technique of this process, called resource optimization techniques. What the project requires on an optimum (8 hours a day, 5 days a week) schedule and what resources are actually available on a day-to-day basis may be two different things, and this technique reconciles the project durations to the reality of resource availability on that particular project.


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