PERT: Program Evaluation and Review Technique

PERT was originally developed for and used on the Polaris missile systems in 1958. It is used in planning and controlling projects where there is uncertainty associated with cost and schedule which must be evaluated in detail. This uncertainty might be due to quantities or productivity being uncertain, on for example, unusual or one off projects.

Like the Critical Path Method (CPM), PERT uses logic diagrams to analyse performance times and focuses on the event. However it estimates the probability of meeting specified completion dates and assumes that activity durations vary based on a varying production rates. The Critical Path Method (CPM) on he other hand assumes that activity durations do not vary.

PERT determines the probability of completing by the contract completion date, by way of a quantified risk assessment based on optimistic, pessimistic and most likely activity durations. The technique helps the scheduler estimate the most probable activity or project duration and is therefore considered a probabilistic method.

Whilst the approach provides a greater level of information to be analysed than deterministic methods such as CPM, it does require multiple time estimates (optimistic, pessimistic and most likely) which can be time consuming to produce. The mathematical calculations are also more complex than other scheduling methods. In addition the analysis is based on the duration of activities along a single path and results can be skewed if there are multiple critical paths on the project. Historically these disadvantages have meant that the technique has not been widely used on construction projects. However more powerful computer applications have made it easier to use the technique which is being used more widely especially as a construction risk analysis technique.