Estimate Activity Duration using Three‐Point Estimates (PERT)

The accuracy of the activity duration estimate can be improved by considering estimation uncertainty and risk.

This concept originated with the Program Evaluation and Review Technique PERT.

The PERT definition is the Program (or Project) Evaluation and Review Technique, commonly abbreviated as PERT. PERT is a statistical tool used in project management designed to analyze and represent the tasks involved in completing a given project. This method helps estimate the confidence level. First developed by the United States Navy in the 1950s, it is commonly used in conjunction with the critical path method (CPM).

PERT (3-Point Estimation Technique) uses three estimates to define an approximate range for an activity’s duration.

Three types of duration could be based on certain conditions or risks. Which one you will choose for your schedule?

Let’s see first what the difference between the three types of durations is.

Most likely Estimate (TM):

The duration of the activity, given the resources likely to be assigned, their productivity, realistic expectations of availability for the scheduled activity, dependencies on other participants, and interruptions.

Optimistic Estimate (tO):

The activity duration is based on a best‐case scenario described in the most likely estimate.

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Pessimistic Estimate (tP):

The activity duration is based on a worst‐case scenario described in the most likely estimate.

NORMAL DISTRIBUTION CURVE

3-Point Estimate Formula (pert formula):

The Most Likely Time (m):

The Optimistic Time (a):
The Pessimistic Time (b):

T exp =(a+4m+b)/6

PERT analysis calculates an expected (tE) using a weighted average of these three estimates:

PERT Program Evaluation Example:

An activity has 3 estimates, provided below:
Optimistic = 10 days
Pessimistic = 30 days
Most Likely/Realistic = 15 days

What does the three-point estimate for this activity?

T (Time)= (10+(4*15) +30)/6 = 16.66 day

16.66 days is the suitable duration to put in your schedule not 10 or 30. This is a professional program evaluation and review technique you can use with the long lead and important activities. I hope you enjoyed this Project Management Three-Point Estimate Technique which is very useful in project scheduling.

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