PERT (Program Evaluation and Review Technique)
DE: PERT (Programmbewertungs- und Ueberpruefungstechnik)
A technique using three estimates (O, M, P) to calculate expected duration.
Detailed Explanation
PERT is a statistical tool used to analyze the tasks involved in completing a project. It uses three estimates — optimistic (O), most likely (M), and pessimistic (P) — to calculate expected duration using the formula: Expected = (O + 4M + P) / 6. Standard deviation = (P - O) / 6.
Originally developed by the US Navy in the 1950s for the Polaris missile program, PERT accounts for uncertainty in task duration estimates. The weighted average gives more influence to the most likely estimate while still incorporating best-case and worst-case scenarios.
PERT is closely related to three-point estimating and is often used as input to Monte Carlo simulation. While PERT assumes a beta distribution for each activity, it provides a quick analytical approach to handling schedule uncertainty without requiring complex simulation software.
Key Points
- Three estimates: Optimistic (O), Most Likely (M), Pessimistic (P)
- Formula: Expected = (O + 4M + P) / 6
- Standard Deviation = (P - O) / 6
- Accounts for uncertainty in duration estimates
- Developed by US Navy for the Polaris program (1950s)
- Often used as input to Monte Carlo simulation
Practical Example
Estimating a data migration task: O = 5 days, M = 8 days, P = 17 days. Expected = (5 + 32 + 17) / 6 = 9 days. SD = (17 - 5) / 6 = 2 days. There is a ~68% chance the task takes 7-11 days and a ~95% chance it takes 5-13 days. The PM uses 11 days (expected + 1 SD) for the schedule to have ~84% confidence.
Tips for Learning and Applying
Get estimates from the people doing the work — not management guesses
The pessimistic estimate should be realistic worst-case, not catastrophic
Use PERT for activities with high uncertainty; deterministic estimates for well-understood work
Combine PERT with Monte Carlo for project-level schedule risk analysis
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