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Time Estimation: A Crash Course

Time estimates can be challenging at best. There are different ways to approach estimates to ensure that it provides enough time to take care of the scope, but also deal with any potential risks.

One way that the PMI has suggested is PERT. PERT stands for Program Evaluation and Review Technique. It involves getting three distinct estimates on the time involved for task. Pessimistic, Optimistic and Most likely; signified by P, O, and M respectively.

The calculation for PERT is

time1

 

 

From the POM estimates we also get the Activity Standard Deviation

time2

 

 

 

and the Activity Variance.

time 3

 

 

We also use the Activity Standard Deviation and the PERT number for The Range of Estimate which is basically PERT +/- Activity Standard Deviation.

When you’re getting estimates from a resource, SEM, or functional manager (ei: Department head or Manager other than a PM), you should press for 3, and plug them into the formula.

Jill is the PM, Dave is the resource.
Jill: Hi Dave, How long do you think it will take you to complete module 1 including clean up, so it’s ready for QA?
Dave: I don’t know.
Jill: I need to know, Can you look into that for me? I need 3 estimates on how long you think it’ll take. The Optimistic estimate if everything runs smoothly, the pessimistic if everything goes pear shaped, and the most likely which is the most realistic. If I come back at 4:30 can you provide me with those numbers?
Dave: Yes.

Jill: Hey Dave, do you have those numbers.
Dave: Yep- Optimistically, it will take 24 man days, Pessimistically, 37 man days. Most likely is 30 man days.

PERT ASD Activity Variance Range
30.167 2.167 4.696 28.0 – 32.334

For more information, please refer to Chapter 6 of the PMBOK Guide.