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What is the PERT three-point estimation formula?

PERT (Program Evaluation Review Technique) uses te = (O + 4M + P) / 6, where O is the optimistic estimate, M is the most likely estimate, and P is the pessimistic estimate. This weighted average assumes a beta distribution and is used when activity durations are uncertain.

What is the standard deviation in PERT and why does it matter?

PERT standard deviation SD = (P - O) / 6. A larger spread between optimistic and pessimistic estimates indicates higher uncertainty. The SD is used to calculate confidence ranges: 68% within ±1 SD, 95% within ±2 SD. This helps quantify schedule risk.

When should you use PERT instead of a single-point estimate?

Use PERT when activity durations are uncertain or when historical data is limited. It is especially useful in research, development, and first-of-a-kind projects. For well-understood, repetitive activities, a single-point (most likely) estimate is sufficient.

What is the difference between PERT and CPM?

CPM (Critical Path Method) uses single-point deterministic estimates to find the critical path. PERT uses three-point probabilistic estimates to calculate expected duration and variance, allowing for schedule risk analysis. PERT is better for uncertain projects; CPM is better for well-defined projects.

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PERT Three-Point Estimate Calculator for Construction Activity Duration

Formula

te = (O + 4M + P) / 6; SD = (P - O) / 6; Variance = SD²

Governing Standard

PMI PMBOK 7th Edition — Schedule Management, and PERT (Program Evaluation Review Technique)

Required Inputs

Optimistic Estimate (O) in days
Most Likely Estimate (M) in days
Pessimistic Estimate (P) in days

Output

Expected Duration (te), Standard Deviation (SD), 68% Range, and 95% Range in days

Notes

For an activity with O=10, M=14, P=26 days: te = (10 + 56 + 26)/6 = 15.3 days. SD = (26-10)/6 = 2.67 days. 68% confidence range: 12.6 to 18.0 days. 95% confidence range: 10.0 to 20.6 days. The PERT formula weights the most likely estimate 4x more than optimistic or pessimistic, reflecting the beta distribution assumption.

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PERT Calculator for Construction Project Duration (3-Point Estimate)