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## 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.