PMP Exam Formula Guide: PERT Explained

PMP Exam Formula questions could make or break your exam result. If you are preparing for the PMP certification you already know, there will be several questions based on the formulas used in project management.
Many times we end up just remembering the formula for the exam purpose and not attempting to understand the underlying principles behind the formula. Without knowing the principles of the formula one  might find it difficult to decide which formula to use.
PM Exam Academy, through a series of articles,is trying to help the PMP aspirants to prepare better for the exam by explaining these PMP Exam  formulas.

PERT Formula Definition

In this article let us discuss PERT, which stands for Program Evaluation and Review Technique.
PERT is used to come up with an estimated time to complete the tasks in the project.
Estimated time Te = ( Tp+ 4(Tn)+To )/6
 
where
 Tp = Pessimistic estimation to complete a task
 To = Optimistic estimation to complete a task
 Tn = Normal estimation to complete a task
PERT is used where there are uncertainties in estimating the time to complete the tasks.
As you see above, the formula gives 4 times more weight to the normal estimation compared to pessimistic and optimistic estimations. If pessimistic and optimistic values are “equidistant” from the normal estimation, Te will be equal to Tn. But when one of them is farther from the normal value, Te will swing towards that value to compensate for the uncertainty.
That way PERT gives a “realistic” value of the estimation, not only by considering the optimistic and pessimistic values( like standard deviation does), but also taking the distance into account.
PERT is a very simple approach to come up with task estimations where Tp,To and Tn could come from expert judgement and past experience. With this information, PERT can easily give you a more realistic value.
By calculating the estimated value for each task using PERT, a project network diagram can be easily prepared.

PERT Example

Do you want to use PERT to see how long will it take to study for PMP exam? Let us do the math.
Let us say Tp = 6 month, To = 1 month  and Tn = 3 months
 
Estimated time, Te = ( 1 +4(3) + 6)/6 = 3.16 month
As you can see estimated time 3.16 months is slightly biased towards pessimistic estimate since it is farther from the normal estimate compared to optimistic estimate.
You can also consider positive and negative risk factors while coming up with To and Tp which will influence the Te value.