PERT chart

A PERT chart is a visual project management tool used to map out and track tasks and timelines. The acronym standard for Project (or Program) Evaluation and Review Technique.

PERT was developed in 1958 by the Special Projects Office of the US Department of Defense as part of the Polaris submarine-launched ballistic missile programme. It built on Critical Path Analysis techniques to support calculations of the time needed to complete projects through systematic analysis of the tasks involved.

In a PERT chart, boxes or circles ("nodes") are drawn to represent events or milestones. These nodes are then connected using arrows, representing the sequence in which tasks must be completed due to dependencies.

A distinctive feature of PERT is its approach to timescale uncertainty. Rather than a single estimate, PERT uses three probabilistic time estimates for each task: optimistic time, most likely time, and pessimistic time. These are combined into a weighted average using the formula:

This gives greater weighting to the most likely time, while still accounting for best- and worst-case scenarios.

PERT charts are similar to [Gantt charts] in that they offer a graphical view of a project’s tasks, schedule, and timelines. PERT charts are more free-form than Gantt charts, allowing for greater flexibility in how tasks are represented. PERT charts are also better for illustrating dependencies.