Enter optimistic, most likely and pessimistic estimates for one or many tasks. Get expected duration, standard deviation, variance and 68/95/99.7% ranges.
| Task | Optimistic (O) | Most likely (M) | Pessimistic (P) |
|---|
Three-point estimating reduces uncertainty by using three scenarios instead of one guess.
E = (O + 4M + P) ÷ 6 – weighted toward the most likely value.E = (O + M + P) ÷ 3 – simple average.σ = (P − O) ÷ 6. Variance: σ².If the question says "PERT" or "weighted average", use the ÷6 formula. If it says "triangular" or "simple average", use ÷3. ±1σ ≈ 68%, ±2σ ≈ 95%, ±3σ ≈ 99.7%.
The weights 1 + 4 + 1 add up to 6, so dividing by 6 gives a weighted average where the most likely estimate counts four times.
Add each task's variance (σ²), then take the square root of the total. Never just add the standard deviations.
Whenever a question gives optimistic, most likely and pessimistic estimates, or asks about duration uncertainty and confidence ranges.