Question

PERT networks use a simple statistical method to determine the most likely task completion times. Describe how expected activity times and variances can be computed in a PERT network. Describe a situation in which a project manager would choose PERT for her project.

Answer 1

PERT (Program Evaluation Review Technique) is a framework for planning, scheduling, monitoring and
controlling projects with many activities or tasks. This framework allows for scheduling of projects where
the activities have uncertain or probabilistic durations.

In a PERT analysis, the expected completion time for each activity, te, is computed as a weighted average
of the optimistic, most likely and pessimistic estimates:

t_e = (a + 4m + b)/6
where a, m and b are the optimistic, most likely and pessimistic estimates, respectively.

The variance of the expected activity time is then simply computed by the following expression:
?² = ((b- a)/6)²

Finally, the variance in the total project completion time is given by the sum of the variances of the
activities on the critical path:
(s_p)² = (s₁)² + (s₂)² + (s₃)² + ...........+ (s_n)²
where s is the variance of task i, and there are n total tasks on the critical path.

Unlike the critical path method (CPM), PERT analysis allows for uncertain time estimates. But beyond
providing estimates for the project’s start and completion times of activities, the expected completion time
of the project, and the variability in the project completion time, PERT can provide estimates of probability
that the project will be completed on time or, conversely, what the project completion time will be given a
certain acceptable level of probability.

As an example, a project manager running a highway construction project would very likely want choose
the PERT framework to manage the job. This is because uncertainties due to weather, supplies and labor
availability would all insert variances into her estimated activity completion times. PERT gives her a
mechanism for handing this uncertainty.

Another example would be a software development effort wherein a company is hired to implement a new
web and mobile enabled customer interface for a bank’s auditing system. In such a case, the project
manager will likely face shifting requirements, causing uncertainty in the various activity times.