Assumptions and method
Assumes independent durations on a fixed path. Alternative paths, correlation and resource constraints invalidate a project-confidence interpretation.
Test a scenario
Use the result in a decision
Check dependencies, duration assumptions and resource availability.
Assumes independent durations on a fixed path. Alternative paths, correlation and resource constraints invalidate a project-confidence interpretation.
Set a baseline and change one assumption to compare outcomes.
A worked example
Follow the fixed teaching example
- Activity means are (3 + 4×5 + 9)/6 and (2 + 4×4 + 6)/6: 5.3333 and 4 days.
- Their sum is 9.3333 days. Sum variances 1 + 4/9 = 1.4444 days², then take the square root for 1.2019 days.
These illustrative inputs describe a project scenario, not a published benchmark. All monetary inputs use the same currency and price basis.
- One activity per line: optimistic, most likely, pessimistic
- 3, 5, 9 2, 4, 6
- Fixed-path mean
- 9.3333 days
- Fixed-path standard deviation
- 1.2019 days
- Variance
- 1.4444 days²
Interpret the output only within the assumptions above. Changing the inputs changes the result; it does not validate the inputs.
Source & credit
D. G. Malcolm, J. H. Roseboom, C. E. Clark and W. Fazar, Application of a Technique for Research and Development Program Evaluation (1959). This calculator uses a simplified fixed-path approximation, not the entire original procedure.
Malcolm, Roseboom, Clark & Fazar (1959): PERT
This is independently written code and explanation of the underlying method. The linked publication, its diagrams and its trademarks remain its owner’s material; no permission to reuse them is implied.