Expected duration and standard deviation per PERT
PERT (Program Evaluation and Review Technique) is used when activity durations have high uncertainty. It is most valuable for R&D projects, first-of-a-kind work, or when historical data is limited. For routine projects with well-known durations, single-point estimates may suffice.
The PERT (Program Evaluation and Review Technique) calculator computes the expected duration and uncertainty of a project activity from three-point estimates. PERT is a key scheduling tool tested on the PMP exam and used in real-world project management for activities with high duration uncertainty. Enter the optimistic, most likely, and pessimistic estimates to get the expected duration, standard deviation, and confidence ranges.
The expected duration is TE = (O + 4M + P) / 6. This weighted average gives the most likely estimate (M) four times the weight of the optimistic (O) and pessimistic (P) estimates, reflecting the Beta distribution that PERT assumes. The standard deviation is σ = (P − O) / 6, which measures the spread of uncertainty.
The calculator shows two confidence ranges: 68% (TE ± σ) and 95% (TE ± 2σ). Project managers use these to set schedule contingency reserves. If the 95% range is 10–25 days for a critical activity, you might allocate contingency near the upper bound. The wider the range between optimistic and pessimistic, the more uncertainty exists — and the more contingency you should reserve.
PERT is used when activity durations are uncertain (R&D, first-of-a-kind projects). CPM (Critical Path Method) uses single-point estimates and is better for routine, well-understood work. On the PMP exam, you may be asked to calculate TE, σ, or the probability of completing within a given timeframe using the normal distribution assumption.
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