Project Evaluation Review Technique (PERT)

Project Evaluation and Review Technique


The Program (or Project) Evaluation Review Technique (PERT) is an extension of the Critical Path method, in which activities involved in completing the project are analyzed. PERT is used when the time of activity is uncertain. Therefore, the time each activity will take to complete is based on the range of probabilities. There are three types of activity times in PERT, which are used to calculate the duration of each activity in the project. They are as follows:

Optimistic Time (O): It is the shortest possible time within which the project can be completed. Every organization wants to achieve this time for the completion of their project.

Pessimistic Time (P): It is the maximum possible time required to complete the activities of the projects.

Most Likely Time (M): The best estimated time required to complete the project under normal circumstances is referred as the most likely time.

Thus, the expected duration of the activity in the project is calculated using the following formula:

= (O + 4M + P ) / 6

Steps in PERT Planning:

The steps involved in PERT planning are as follows:

Identify Activities and Events: The first step is to identify the activities required to complete the project and the events that mark the beginning and the end of each activity.

Determine the Proper Sequence of the Activities: The next step is to arrange the activities in order to determine the relationship between them.

Construct the Network Diagram: The next step is to construct the network diagram using the above-mentioned steps. In PERT network diagram, some additional information is also included for each activity, which is as follows:

  • Early Start (ES): It is the earliest time at which the activity can begin. It is the earliest finish time for the immediately preceding activity.
  • Early Finish (EF): It is the earliest time at which the activity can end. It is calculated as:

EF = ET + duration of activity

  • Late Start (LS): It is the latest time at which the activity can begin without allowing any delay in its completion. It is calculated as:

LS = LF – duration of activity

  • Late Finish (LF): It is the latest time at which the activity can end, thereby allowing the project to be completed in time. It is the latest start time of the activity that immediately follows it.
  • Slack: It is the amount of time for which the activity can be delayed without impacting the duration of the project. It is calculated as:

Slack = Late start — Early start or

Slack = Late finish — Early finish

  • Estimate the Time Required for Each Activity: The next step is to determine the activity duration on the basis of the three probabilities — optimistic, most likely and pessimistic.
  • Determine the Critical Path: This step determines the longest path in the network and finds the duration of the path.
  • Update the PERT Chart: As the project progresses, the estimated time can be replaced with the actual time of various activities. 

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