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CHAPTER 3.2

CRITICAL PATH, CRITICAL
CHAIN, AND UNCERTAINTY

Exploring Concepts of Shared Contingency

In the 45 years that formal critical path scheduling has been around, all the pop-
ular protocols essentially dealt with uncertainty in a similar way. They addressed
it (if at all) on a task by task basis. The original PERT method did have a formal
mechanism for uncertainty, by providing for three task durations (optimistic, most
likely, and pessimistic). Some of today’s CPM programs have carried over this
three-duration capability (see Chapter 6.3). For those that did not employ the
three-duration approach, uncertainty was dealt with by sneaking in a bit of extra
time in each task duration estimate.

There are several problems with this approach. In the case of the latter, hap-
hazard approach, there was no consistency in the treatment of the schedule con-
tingency allowance, and there was no documentation of what part of the duration
was actual contingency. In all the approaches, the contingency (allowance for un-
certainty) was doled out to each individual task, although the actual uncertainty
would be better addressed on a group of tasks basis.

This situation has been explored by several individuals, and there is a growing
interest in some emerging treatment of (what I call) shared contingency.

Trap Schedule contingency is a vital component of a success-
ful project. However, this contingency must be clearly identi-
fied and managed. Inconsistent and unstructured padding of

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