Critical Chain Theory

Eliyahu Goldratt wrote several books that explain his ideas. Since he is now mentioned in the Guide to the PMBOK, it might be worthwhile to read about his work. He has written a series of novels—The Goal, It's Not Luck, and Critical Chain—that explain many of his ideas. Since they are written as novels, it is rather pleasant reading and a painless way to learn some new ideas about managing and project management.

His critical chain theory is relevant to scheduling. The first idea is that a real project's resource constrained schedule will usually have a large number of activities that have reasonably large amounts of float. Unlike the types of schedules that we see in the classroom, real project schedules will have several hundred activities. Real projects will also have many activities on the critical path that are buffered or resource constrained causing a lengthening of the project completion date. This in turn creates float and free float in the activities not on the critical path.

Referring to Figure 2-15, you can think of the series of activities that make up the critical path (those that have zero float or close to zero float) as the critical chain activities. The activities not on the critical chain generally fall into groups of activities that are dependent on each other but as a whole are done independent of the critical path activities until one of them joins the critical path. That is, until an activity has a critical path activity dependent on it. These groups of activities are called feeder chains.

Normally most projects are scheduled according to the early schedule dates or the early schedule. When this is done the float for the feeder chains collects at the end of the feeder chain. When this is done the last activity in the feeder chain will be completed long before it joins the critical chain of activities. Goldratt says that this is a mistake because any knowledge gained as a result of experience and risk events in activities on the

Figure 2-15. Feeder chains and critical chains.

Figure 2-15. Feeder chains and critical chains.

A

B

C

D

Tot.

Expected value

SC

2C

25

10

85

Standard deviation

S

S

4

2

6.2

critical path will not benefit the activities on the feeder chains. By delaying the start schedule for these activities we can take advantage of the knowledge that is gained working on the critical activities.

Of course if we delay the start of the feeder chain activities too much, they will become part of the critical chain and this is not good. The feeder chains must be buffered to be completed two standard deviations before their late finish dates. Of course, if we expect to make the promise date for our customer, the critical chain activities must be buffered two standard deviations as well, and this must be done before buffering the feeder chains (see the section on buffering schedules earlier in this chapter).

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