Expected Duration

In the PERT methodology, the Expected Duration (E) is the calculated, statistically weighted estimate of how long a task is likely to take. It's derived from the Optimistic (O), Most Likely (M), and Pessimistic (P) estimates derived during PERT.

The specific formula used is: E = (O + 4M + P) / 6

This isn't just a simple average. It's a weighted average. The formula deliberately gives the Most Likely (M) estimate four times the weight of the Optimistic (O) and Pessimistic (P) estimates. The rationale, often linked to the statistical Beta distribution, is that the 'most likely' scenario is inherently more probable than the best or worst possible extremes. This weighting pulls the resulting 'E' value towards your most realistic assessment (M) while still mathematically incorporating the potential impact of the optimistic or pessimistic outcomes.

For example, with O=3, M=6, P=15 days:
E = (3 + 4*6 + 15) / 6 = (3 + 24 + 15) / 6 = 42 / 6 = 7 days.

The result is a single, statistically-informed figure that accounts for uncertainty, making it a more reliable input for planning than a simple guess or an unweighted average.