Program evaluation and review technique (PERT)

PERT is a time-event network analysis system in which various events in a program or project are identified with a planned time established for each. These events arc placed in a network showing the relationship of each event to the other events.

This example illustrates the basic nature of PERT. Each circle of the chart represents an event a supporting plan whose completion can be measured at a given time. The circles are numbered in the order in which the events occur. Each arrow represents an activity-the time consuming clement of a program, the effort that must be made between events. "Activity time", represented by the numbers beside the arrows, is the elapsed time required to accomplish an event.

 It is to be remembered that an accurate estimate of time is difficult. When several estimates arc made, they are usually averaged, with special weight given to the most likely estimate; a single estimate is then used for calculations.

The next step is to compute the Critical Path, that is, the sequence of events which takes the longest time and which has zero (or the leas() slack time. In Figure 15.2. the critical path comprises events 1-34-8-9-13. Over this path, the activity time for this sequence of events is 131.6 weeks. If promised delivery is in 135 weeks, even this critical path will have been completed 3.4 weeks ahead of time. Some of the other paths are almost as long as the critical path. For example, the path 1-2-9-13 is 129:4

weeks. This is not unusual in PERT charts, and it is customary to identify several critical paths in order of importance. Although the critical path has a way of changing as key events arc delayed in other parts of the program, identifying it at the start makes possible the close monitoring of this particular sequence of events to ensure that the total program is on schedule. A PERT analysis, involving large numbers of events, is usually handled with computers.