PERTdiagram

A PERT diagram, or PERT chart, is a planning and scheduling tool used to model the tasks involved in a project and the relationships among them. It emphasizes the sequence of activities and the uncertainties in their durations, helping to estimate total project duration and identify critical work items. Traditional PERT diagrams depict events as nodes and activities as directed arrows, though some variants place activities on nodes.

Key components of a PERT diagram are activities, events (milestones), and the precedence relationships that show which tasks must precede others. Each activity has a duration estimate, and for many PERT analyses, three time estimates are collected: optimistic, most likely, and pessimistic. These three values feed into a probabilistic time estimate that underpins the scheduling calculations.

A central concept in PERT is the critical path, the longest-duration path through the network. This path determines the earliest possible completion time for the project. By calculating earliest start and finish times, latest start and finish times, and slack (float) for each activity, project managers identify tasks that cannot slip without delaying the project. PERT often uses the beta distribution to derive a single expected duration for an activity, Te = (a + 4m + b) / 6, and a corresponding standard deviation of (b − a) / 6 to reflect uncertainty.

PERT diagrams are commonly contrasted with the deterministic Critical Path Method (CPM), which uses fixed activity durations. PERT is particularly suited to research, development, and other projects with high time uncertainty, while CPM is favored for routine, repetitive projects with well-defined task times.

Limitations include reliance on estimates, potential for overly complex networks, and assumptions of independent task durations and accurately defined dependencies.