Program Evaluation and Review Technique
PERT (Program Evaluation and Review Technique) estimates activity durations probabilistically using three points — optimistic, most likely and pessimistic — to produce a weighted expected duration and a measure of uncertainty.
PERT is a scheduling and estimation technique that handles uncertainty by asking, for each activity, three questions: how long if everything goes well (optimistic, O), how long normally (most likely, M), and how long if things go badly (pessimistic, P). It combines them into a weighted average — the expected time — that leans toward the most likely value but accounts for the tails. PERT was developed for the US Navy’s Polaris programme, where activity durations were genuinely unknown, and it pairs naturally with the critical path method.
Program Evaluation and Review Technique at a glance
- Category
- Methodologies & Frameworks · Scheduling & Time Management · Tools & Techniques
- Type
- Methodology / framework
- Also known as
- PERT, PERT Analysis
- Appears in
- 3 sections
- Related
- Critical Path Method, Theory of Constraints, Critical Chain Project Management
Also known as: PERT, PERT Analysis.
Why it matters
Single-point estimates ("this takes 6 days") hide the uncertainty that actually drives project risk. PERT makes that uncertainty explicit and quantifiable: the standard deviation tells you how confident to be, and summing variances along the critical path lets you state a completion date with a confidence level (for example, an 80% chance of finishing by a given date) rather than a single optimistic number that is almost always wrong.
When to use it
Use PERT when activity durations are uncertain and the cost of being wrong is high — research, new-technology, first-of-a-kind work. It adds effort (three estimates instead of one) so it is overkill for routine, well-understood tasks. Many teams apply it selectively to the riskiest activities rather than the whole plan.
How to use it
- For each activity, gather three estimates: Optimistic (O), Most Likely (M) and Pessimistic (P).
- Compute the expected duration: TE = (O + 4M + P) ÷ 6.
- Compute the standard deviation: SD = (P − O) ÷ 6, and variance = SD².
- Use TE values to find the critical path.
- Sum the variances of critical activities, take the square root for the path’s SD, and use it to state completion confidence.
Example
An activity is estimated at O = 4, M = 6, P = 14 days. TE = (4 + 24 + 14) ÷ 6 = 7 days. SD = (14 − 4) ÷ 6 ≈ 1.67 days. So you would plan 7 days but recognise a realistic spread of roughly 3.7 to 10.3 days at two standard deviations.
Template
A PERT worksheet captures O, M and P per activity and auto-calculates TE, SD and variance — see the calculator below.
Tools
Formula & calculator
Try it yourself: PERT Calculator computes this from your own figures.
FAQs
Why is the most likely estimate weighted by 4?
What is the difference between PERT and CPM?
Is PERT the same as three-point estimation?
Alternatives
- Critical Path Method — deterministic durations
- Monte Carlo simulation — models the full distribution, not just three points
- Story-point / velocity estimation — the Agile alternative