Bluejeans’ Place
A Non-profit Educational Website
Dedicated to
Blues Music, Civil Rights History, Project Management and More
Pat Shediack, 5123 Mallet Club Drive, Dayton OH 45439, pat@bluejeansplace.com
EQUATION FOR USING
THE PROGRAM EVALUATION AND REVIEW TECHNIQUE (PERT)
TO ESTIMATE WEIGHTED AVERAGE DURATION OF TASKS
The PERT method uses sequential network logic and a weighted average duration estimate to calculate project duration. Although there are surface differences, PERT differs from the Critical Path Method (CPM) in that it uses the distribution’s mean (expected value) instead of the most likely estimate originally used in CPM.
PERT itself is seldom used today although PERT-like estimates are often used in CPM calculations.
The PERT Equation is:
(
Optimistic Estimate + 4 Times Most Likely Estimate + Pessimistic Estimate
6
)
(source: Project Management Body of Knowledge text, Para 6.4.2.2 and PMBOK text,
Figure 6-4)
Example:
(100 + (4)150 + 250
6
= 950
6

Bluejeans’ Place
A Non-profit Educational Website
Dedicated to
Blues Music, Civil Rights History, Project Management and More
Pat Shediack, 5123 Mallet Club Drive, Dayton OH 45439, pat@bluejeansplace.com
= 158
158 is the weighted average estimate
FOR PMP EXAM CANDIDATES: The idea of PERT, but not the equation, is on the PMP examination test. The key knowledge items are: PERT uses weighted average duration estimates, PERT isn’t used much anymore, but PERT-like estimates (such as the above use) are used often in CPM calculations.
Practical usages:
Looking back at our example, 100 could be your “early finish date,” 250 could be your “late finish” (also on your critical path!) and 158 could be the desired finish date.
Another use would be to use 100 as your “early finish date”, 158 as your “late finish” and the difference between 158 and 250 could be your buffer time if you told the people providing estimate inputs to include buffer time in their numbers.
You could also consider your PERT weighted average estimate your “early finish” and your
“most pessimistic” numbers as your “late finish” which equates to your critical path dates.
What remains between these two numbers would then create your float.
And to answer the “so what” question…You could take your PERT estimate number and plug it into your cost variance and schedule variance equations to determine where your project stands in relation to time and funding budgets prior to finalizing a related document (e.g., proposal).
* * * * *
Compiled as a PMP test preparation study aid by Pat Shediack, PMP, http://www.bluejeansplace.com
, January
11, 2000 Updated August 28, 2002  Patrick D. Shediack 1999-2007 All Rights Reserved other than those sections quoted from the PMBOK that is copyrighted by PMI. This document may be used without compensation to the copyright holder providing full credit and web site sourcing is given and no commercial or for profit gain is accrued other than to the copyright holder.