The accuracy of single-point activity duration estimates may be improved by considering estimation uncertainty and risk. This concept originated with the program evaluation and review technique (PERT). PERT uses three estimates to define an approximate range for an activity’s duration:

•  Most likely (tM). This estimate is based on the duration of the activity, given the resources likely to be assigned, their productivity, realistic expectations of availability for the activity, dependencies on other participants, and interruptions.

•  Optimistic (tO). The activity duration based on analysis of the best-case scenario for the activity.

•  Pessimistic (tP). The activity duration based on analysis of the worst-case scenario for the activity. 

 

Depending on the assumed distribution of values within the range of the three estimates the expected duration, tE, can be calculated using a formula. Two commonly used formulas are triangular and beta distributions. The formulas are:

 

Triangular Distribution. tE = (tO + tM + tP) / 3

Beta Distribution (from the traditional PERT technique). tE = (tO + 4tM + tP) / 6

 

 

 

 

Duration estimates based on three points with an assumed distribution provide an expected duration and clarify the range of uncertainty around the expected duration.