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The Program Evaluation and Review Technique, known as PERT, is a project management tool that maps every task in a complex project as a network of activities, estimates how long each will take using a three-point probability formula, and identifies which sequence of tasks, the critical path, determines the project's earliest possible completion date. It was developed in 1957 by the U.S. Navy Special Projects Office, working with Lockheed Aircraft and Booz Allen Hamilton, specifically to manage the development of the Polaris nuclear submarine missile system.
PERT is the project manager's way of converting uncertainty into a probability-based schedule rather than a hopeful guess.
The core innovation of PERT is that it accepts uncertainty in task duration rather than demanding a single fixed estimate. For every activity in the project, you record three time estimates.
PERT combines these into an expected time using a weighted average formula that gives the most likely estimate four times the weight of the extremes: (O + 4M + P) / 6. This formula, derived from the beta distribution, produces an expected duration with a known statistical confidence level.
Once expected durations are calculated, PERT maps every possible sequence through the network from project start to project finish. The critical path is the longest sequence, the one that determines the earliest finish date. Any delay on a critical path activity delays the entire project by the same amount.
Non-critical activities have slack, the amount of time by which they can be delayed without pushing back the project completion date. A task with two weeks of slack means you can reassign its resources temporarily or allow it to start late without consequence. PERT makes this visible so managers can allocate resources where they matter most: critical path activities with zero slack.
PERT and the Critical Path Method are often used together but serve different purposes. PERT is probabilistic: it is built for projects where task durations are uncertain, such as research, product development, or construction in complex environments. The Critical Path Method is deterministic: it assumes each task has a single fixed duration and focuses on cost and schedule optimization for projects where durations are well understood, such as routine construction or manufacturing ramp-ups.
In practice, modern project management software combines both approaches. You enter optimistic, most likely, and pessimistic estimates for uncertain tasks and single-point estimates for well-defined ones, letting the software compute the critical path and schedule variance automatically.
A PERT chart is a network diagram, not a Gantt bar chart. Numbered nodes represent project milestones or completed phases. Arrows connecting the nodes represent activities, with the expected duration labeled on each arrow. Multiple arrows from a single node show parallel activities that can proceed simultaneously. The critical path, the longest chain from start to finish, is typically highlighted in red.
The visual structure makes dependencies visible at a glance. If Task C cannot begin until both Task A and Task B are complete, the PERT chart shows two arrows converging on the node that starts Task C. A Gantt chart shows the same information in a bar format but makes these dependencies harder to read in complex projects.
PERT requires accurate time estimates, which are difficult to provide for truly novel projects. If estimators are systematically optimistic, every estimate will be low, and the critical path calculation will produce an unrealistically early finish date. PERT also does not address resource constraints: two critical path activities might both require the same specialist, but PERT does not signal the conflict unless a resource loading analysis is performed separately.
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