Program Evaluation Review Technique (PERT)

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Program Evaluation Review Technique (PERT)

A Program Evaluation Review Technique (PERT) chart is a graphical project-management tool that maps a project’s timeline by listing all tasks (events or milestones), their durations, and the dependencies among them. PERT helps managers estimate the time and resources needed, identify the critical path, and evaluate schedule risks.

Origins

PERT was developed in the 1950s by the U.S. Navy’s Special Projects Office to manage the Polaris submarine project. Its use of probabilistic time estimates made it useful for complex, uncertain projects.

Main components

  • Nodes (circles or rectangles): represent events or milestones.
  • Vectors/Arrows: link nodes to show tasks and their sequence.
  • Task durations: time estimates associated with tasks (often using three-point estimates).
  • Dummy activities (dotted lines): show logical relationships without actual work or time.
  • Critical path: the longest sequence of dependent tasks that determines the minimum project duration.

How PERT works

  1. Break the project into discrete tasks or milestones.
  2. Determine dependencies (which tasks must precede others).
  3. Estimate task durations—traditionally using three time estimates:
  4. Optimistic time (O): shortest possible duration.
  5. Most likely time (M): best estimate under normal conditions.
  6. Pessimistic time (P): longest possible duration.
  7. Calculate the expected time (TE) for each task. Common PERT formula:
    TE = (O + 4M + P) / 6
  8. Draw the network: nodes connected by arrows according to dependencies; include dummy activities where needed.
  9. Identify the critical path—the sequence of tasks with no slack that sets the project’s minimum completion time.
  10. Update the chart as the project progresses.

Interpreting a PERT chart

  • Arrow direction shows workflow and sequence.
  • Parallel tasks are independent tasks that can run simultaneously.
  • Tasks on the critical path cannot be delayed without delaying the project.
  • Dotted arrows (dummy activities) clarify sequencing when two paths intersect but no actual task exists.

Advantages

  • Clarifies task dependencies and project logic.
  • Supports probabilistic time estimates, which helps account for uncertainty.
  • Highlights the critical path so managers know where to focus monitoring and resources.
  • Encourages cross-departmental input and improves communication.
  • Useful for what-if analysis (evaluating schedule impacts of changes).

Disadvantages

  • Requires significant effort to build and maintain accurately.
  • Time and cost estimates can be subjective and unreliable.
  • Focuses on scheduling; may not show financial or resource constraints in detail.
  • Complexity can make charts difficult to interpret for large projects.

PERT vs. Gantt

  • PERT: Network diagram emphasizing task dependencies and probabilistic time estimates; good for planning and analyzing complex dependencies.
  • Gantt: Bar chart showing start/end dates and durations; widely used for scheduling and tracking execution.
  • Common practice: use PERT for planning (to discover dependencies and critical path) and Gantt charts for day-to-day schedule monitoring.

Creating a PERT chart (tools and steps)

Tools:
* Spreadsheet software (e.g., Excel)
* Diagramming apps and templates (e.g., Lucidchart, Microsoft Visio, project-management software)

Basic steps:
1. List tasks and deliverables.
2. Determine dependencies and responsible parties.
3. Estimate O, M, and P times (and compute TE).
4. Draw nodes and connect them with arrows; include dummy activities if needed.
5. Compute earliest/latest start and finish times and identify the critical path.
6. Review and update regularly.

Example use case

In the 1950s Polaris program, PERT helped managers evaluate schedule validity, track progress, and assess effects of changes—demonstrating its value for complex, multi-contractor projects.

Key takeaways

  • PERT is a planning-focused network diagram that uses probabilistic time estimates to model project schedules.
  • It exposes dependencies and the critical path, helping prioritize monitoring and resource allocation.
  • PERT is powerful for planning but requires careful, ongoing maintenance and reliable estimates to be effective.