152 Project Management
However, despite your best efforts to identify and address
threats, a large number of circumstances (think of them as little
threats) will remain that you won’t be able to identify or have
the resources to address. Further, too many things can happen
that you simply cannot foresee or predict. That’s why variability
is inherent in projects.
This inherent variability is impossible to manage away.
Therefore, you must acknowledge it and accommodate it.
Recognize, evaluate, estimate, and communicate its existence
to your management and other stakeholders, as appropriate.
This module offers some insight on methods for accommodat-
ing uncertainty and the inherent variability that goes with it.
With luck (and statistics) on your side, the positive and negative
variability will even out and you’ll end up somewhere near your
estimated project targets.
Using “PERT” Calculations to Determine Schedule Durations
The calculations used in the Program Evaluation and Review
Technique (PERT) approach recognize the variability inherent in
each activity and applies rudimentary statistics in a way that
accommodates the variability. PERT calls for three estimates to
be provided for each activity:
- Pessimistic:the duration if things go poorly
- Optimistic:the duration if things go very smoothly
- Most Likely: our “best guess”
Figure 8-5 offers more insight on this technique for accom-
modating uncertainty.
PERT An acronym for Program Evaluation and Review
Technique. Many people erroneously refer to the network
diagrams with lines and bubbles as “PERT charts,” believing
that the bubbles are what make that particular network diagram a
PERT chart.What distinguishes the PERT approach from other net-
work diagramming techniques is the use of a probabilistic approach.
PERT uses statistics to determine activity durations and to calculate
the probabilities of specific project outcomes.