PERT

Details
Full Name

Program Evaluation and Review Technique

Also known as

Three-Point Estimation, PERT Network Analysis

Core Concepts:

Three-Time Estimation

Each activity requires three duration estimates: Optimistic (O) — best case if everything goes well; Most Likely (M) — realistic expected duration; Pessimistic (P) — worst case if problems arise

PERT Formula

Weighted average: E = (O + 4M + P) / 6 — weights the most likely estimate four times more than the extreme estimates

Standard Deviation

σ = (P − O) / 6 — quantifies schedule uncertainty per activity; used to calculate confidence intervals for the project completion date

Network Diagram

Activities are represented as nodes or arrows in a directed acyclic graph; dependencies between activities are modelled explicitly

Critical Path

The longest path through the network determines the minimum project duration; any delay on the critical path delays the whole project

Float / Slack

Time an activity can be delayed without affecting the project end date; activities on the critical path have zero float

Probabilistic Scheduling

Unlike CPM (which uses single-point estimates), PERT treats duration as a random variable, enabling statistical confidence ranges for milestones

Origin

Developed in 1958 by D.G. Malcolm, J.H. Roseboom, C.E. Clark, and W. Fazar at Booz Allen Hamilton for the US Navy’s Polaris submarine missile programme

Key Proponents

D.G. Malcolm, J.H. Roseboom, C.E. Clark, W. Fazar ("Application of a Technique for Research and Development Program Evaluation", 1959)

When to Use:

  • Estimating effort and schedule for projects with significant uncertainty

  • Planning R&D, software development, or any novel work where duration is hard to predict

  • Communicating schedule risk and confidence levels to stakeholders

  • Identifying the critical path to focus risk management and resource allocation

  • Complementing Story Points or T-shirt sizing with a probabilistic time dimension