PERT Estimator

PERT Formula Explained for the PMP Exam

The PERT Expected Duration Formula

t_E = (O + 4M + P) / 6

PERT (Program Evaluation and Review Technique) uses three estimates to calculate a weighted average. The most likely estimate (M) is weighted 4 times more heavily than the optimistic (O) and pessimistic (P) estimates. This creates a beta distribution that accounts for uncertainty while biasing toward the most probable outcome.

Standard Deviation

σ = (P − O) / 6

Standard deviation measures the spread of uncertainty in your estimate. A larger gap between optimistic and pessimistic means more risk. On the PMP exam, standard deviation is used with confidence ranges: ±1σ gives about 68% confidence, ±2σ gives about 95%, and ±3σ gives about 99.7%.

When the PMP Tests This

PERT questions on the 2026 PMP exam typically appear in two forms. First, straightforward calculations: “Given O=3, M=5, P=13, what is the expected duration?” Second, confidence range questions: “What range gives 95% confidence?” (answer: t_E ± 2σ). Know both.

Common Exam Traps

Trap 1: Confusing PERT with triangular estimate. PERT = (O + 4M + P) / 6. Triangular = (O + M + P) / 3. The PMP tests both. Read which one the question asks for.

Trap 2: Variance vs. standard deviation. Variance = σ² (standard deviation squared). If the question asks for variance, square the standard deviation. Some questions give you activities on the critical path and ask for the total project variance — add the individual variances, not the standard deviations.

Trap 3: Adding standard deviations. You cannot simply add standard deviations across activities. You must add the variances, then take the square root to get the combined standard deviation. This is a frequent PMP exam trap for critical path duration confidence questions.

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