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THE PERT METHOD 183

Advanced Time Contingency Methods

Most project managers seem to agree that the most common weakness of proj-
ect schedules is the task estimates. We have trouble estimating the duration of
tasks, as well as the effort required to execute the tasks. There are volumes of
writings on the problems of task estimating, and there would be considerably
more published on the subject, if anyone had any really good solutions for the
problem of estimating.

There are two well-accepted (but competing) structured methods for dealing
with the fallacy of task estimating and the tendency to inject schedule contin-
gency into the task estimates. The older, well-established method is often called
the PERT method. A newer approach is called the critical chain method. Both
have a strong following and are effective ways of quantifying schedule contin-
gency within a structured planning environment.

In both cases, we avoid fuzzy, undocumented schedule contingency and create
a measurable and manageable basis for improved time estimating.

The PERT Method

This concept relies on three time estimates per task, rather than a single estimate.
You won’t find the three time estimate approach to be in great demand. After all,
if we have such a terrible time arriving at a reasonable single time estimate, won’t
the PERT approach just give us a very precise error? This is certainly possible,
and we have to evaluate the justification for this estimating mode on a case basis.
On the other hand, it is often easier to provide three estimates for which the basis
of the estimate is clear, than one estimate that considers multiple scenarios and
blurs the basis for the calculation.

Given the softness in our base estimates, what do we gain from the triple esti-
mate approach? First, we are more likely to gain precision in the time estimates.
When we ask a performer to estimate the duration of the task, we often get a bi-
ased estimate. The performer may be overly optimistic, assuming that everything
will go well (Murphy is on someone else’s job). Or the performer may be afraid to
make a commitment based on a best guess so he adds a little time as a safety fac-
tor. So just what does 10 days mean? Is it 10 days if everything goes well, but
more likely to be 13 days? Or is it most likely to be 8 days, but we’ll add a couple
of days as a cushion?

With the PERT approach, we can ask for three distinct time estimates. An op-
timistic estimate is usually a duration that would be achievable about 10 percent
of the time. Likewise the pessimistic estimate is usually a duration that would oc-
cur about 10 percent of the time. The third estimate is the most likely, which we