Planning estimates for scheduling and controls purposes; actual outcomes depend on project execution and management judgment. Aligned with PMI PMBOK & AACE 10S-90 standards.
PERT Three-Point Duration Estimator
Calculate expected weighted duration te = (O + 4M + P)/6, standard deviation σ, variance σ², and target completion probabilities per AACE 10S-90.
Three-Point Estimate Inputs
PERT Weighted Duration Results
te = 6.0 daysPERT Beta Distribution Curve
Frequently Asked Questions (FAQ)
What is the PERT weighted average duration formula?
Expected Duration (Te) = (Optimistic + 4 × Most Likely + Pessimistic) / 6.
What is the formula for task standard deviation (σ) in PERT?
Task Standard Deviation (σ) = (Pessimistic Duration - Optimistic Duration) / 6.
How is PERT task variance (σ²) calculated?
Task Variance is the square of standard deviation: σ² = [(Pessimistic - Optimistic) / 6]².
How do you calculate total project variance on a PERT critical path?
Total Project Variance = Sum of variances (σ²) of all activities lying on the critical path.
How do you calculate the probability of completing a project by a target date?
Compute Z-score: Z = (Target Date - Total Expected Duration) / √(Total Project Variance), then look up Z in a standard normal distribution table.
Why does PERT use a beta distribution weighting of 4 for the Most Likely estimate?
The beta distribution assumes the Most Likely duration occurs most frequently, giving it 4 times the weight of extreme optimistic and pessimistic boundaries.
What is the difference between PERT and CPM?
PERT handles uncertain task durations using three-point probabilistic estimates, whereas CPM uses deterministic single-duration estimates.
What is Triangular distribution in 3-point estimating vs PERT Beta?
Triangular estimation uses Te = (Optimistic + Most Likely + Pessimistic) / 3, assigning equal weight to all three points when historical data is lacking.
How does PERT handle optimistic bias in construction estimation?
By explicitly requiring estimators to define a worst-case pessimistic scenario (95th percentile delay), PERT counteracts optimistic bias.
What confidence level corresponds to +1, +2, and +3 standard deviations in PERT?
In a normal curve: ±1σ = 68.3% probability, ±2σ = 95.4% probability, and ±3σ = 99.7% probability of completing within the time range.