12. PERT

Short for Program Evaluation and Review Technique.

One of the weaknesses of the standard CPA technique is that it assumes that each activity will certainly be acheived in the stated time.

The PERT method extends the standard CPA method by trying to make a more realistic estimate of the time each activity will take.

This is based on first of all stating a very optimistic time for the activity, where the likelyhood of acheiving it in that time is less than 1%. Then consider a very pessimistic estimate of the time, where you are almost certain the activity will be acheived in that time. Then finally, state a time that you think is the most likely time it will take to complete. To summarise

• Most optimisitic time (1% chance)
• Most pessimistic time (almost certain)
• Most likely time (high confidence)

Now use the formula below to work out the estimated time for the activity.

Estimated time = shortest time + (4 x likely time) + longest time

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This is a weighted estimate formula that tends to push the estimate away from the shortest time. This estimated time is applied to each path in the CPA chart.

As the project progresses, the estimated time is replaced with the actual time.

This may cause the critical path to change due to an unexpected delay. In which case, if there is slack in the project, perhaps some of those resources can be deplayed to the critical path.