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.

Pert Definition According to Wikipedia The Program (or Project) Evaluation and Review Technique, commonly abbreviated PERT, is a statistical tool, used in project management, that is designed to analyze and represent the tasks involved in completing a given project. This method help estimates to 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.

There are three types of duration which could be based on some certain conditions or risks, which one you will choose for your schedule? Lets see first what is the different between the three type of durations :

Most likely Estimate (tM):

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

Optimistic Estimate (tO):

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

Pessimistic Estimate (tP):

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

NORMAL DISTRIBUTION CURVE

3-Point Estimate Formula (pert formula):

The Optimistic Time (a) :
The Most Likely Time (m):
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
Realistic = 15 days

What is 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 nor 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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