PERT - Project Management Techniques

What is PERT Chart?

A Program Evaluation Review Techniques (PERT) chart is a project management tool used to schedule, organize, and coordinate tasks within a project. It is a methodology developed by the U.S. Navy in the 1950s to manage the Polaris submarine missile program. A similar methodology was developed at the same time for Private Sector was Critical Path Method (CPM).

Where has it been helpful?

While Critical Path Method deals with a certainty of duration. PERT deals with uncertainty. It tries to answer a question such as

Expected Project Completion Time?
What is the probability of completing on or before certain time period?
Critical Path Activity that directly impacts the completion time
Activity Start and End Dates

What are the Limitations?

Activity Time Estimated is somewhat subjective and depends on a judgment
Even if Activity Time is well established, PERT assume BETA distribution these time estimates but the actual distribution may be different.
It assumes Probability distribution of the project completion time is the same as that of a critical path.

Steps to Create PERT Diagram

Determine the tasks required to complete the project and the sequence they must be done
Construction network diagram with arrows representing activities and nodes being milestones.
Determine the Critical Path
Estimate the Time required for each activity
Calculate the variance of each Task or Activity
Calculate the Project Variance
Calculate the Standard Deviation of Project Variance
Find of Z Score (Normal Distribution Table)
Update the PERT Chart as project progress

Calculating Estimated Time

Of all the steps, one crucial steps that involve in creating PERT is finding Estimated Time. Considering PERT deals with an uncertain time period, it will involve finding Optimistic Time, Most Likely Time and Pessimistic Time.

Optimistic time is the shortest time assumed for a given task or activity

Pessimistic time is the longest time period assumed for a given task or activity

Most Likely time is the most likely time period it will take to complete a task

Estimated Time is then worked out by = (Optimistic Time + 4 Most Likely Time + Pessimistic Time)/6

Calculate Variance

Considering we are now working with 3 time period – optimistic, most likely, pessimistic time to arrive at estimated time. We need to know what the variance of the time period. Variance is the difference between optimistic and pessimistic time from the mean time period.

[(Pessimistic Time – Optimistic Time)/6]2

Calculate Variance of the Project

In order to calculate the variance of the overall project, add up the variances of the activities under critical Path.

Say for example Critical Path is Activity A, C, E, H. Then add up the variances of these activities to calculate the project variance.

Let’s presume this is 0.89

Calculate Standard Deviation

Standard Deviation is worked out by finding out the square root of the project variance

√0.89 = 0.94

Now to answer our question, “What is the probability of completing on or before certain time period?

Let’s presume Due Date is 12 Weeks
Expected Time of Completion is 11 Weeks
Z Value = (Due Date – Expected Time of Completion)/ √89. It comes up as 1.06
Find the value of 1.06 in a normal distribution table, you will have the value 0.855

Which means 85%. Or the chances of our project getting completed in our expected time duration is 85% and not getting in our time duration will be 15%.

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PERT – Project Management Techniques