My PERT ThreePoint Custom Function

From Dwayne Wright PMP
Certified FileMaker Developer

TWITTER: dwaynewright
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So my big 2010 challenge is getting my PMP (project management professional) certification. This is all part of a “OMG the economy is in the ditches and I only have FileMaker skills’ neurosis and leading to my attempts to build a career backup plan. This posting isn’t about that but just some header info (because I’ll probably be writing a lot about project management calculation functions in the future).

Anyhoo ...

One of the blessed expected value estimation calculation formulas for the PMP is the three point estimate. What it does is take three estimation points and gives you a most likely result. It can be used to calculate time for schedules or money for budget estimates.

Here you can see three values and their three point result.

Here is a probably a better illustration. Say that a client is asking for an estimate of hours to do a project task. You have done this type of work before and have code you can apply to it. You feel pretty confident that you can do the job in 50 hours and perhaps as low as 40. You want to throw in a higher estimate because you might not have all the deliverable scope because the client isn’t that sure about their definitive needs. So you put in a high estimate of 120 hours into the mix.

You could tell the client that it could take anywhere from 40 to 120 hours. This might be a way to get them to better define their scope to you. It also may be a way to ensure that you never hear from the client again. A three point estimate says that if you give them an estimate of 60, you are pretty well covered.

Now every project is unique and no formula is bullet proof but this is one of them that is blessed by the Project Management Institute for PMP certification. Whether or not you are going to be giving the results of the three point estimate to your clients, it might be a worthwhile practice to track it internally. In fact, you could go more granular in this area and have three point estimates for each work package. That way you can refine your estimation skills and build your library of development related organizational process assets!

The custom function is ruthlessly simple. We take ...

- the most likely estimate times four
- add to that the optimistic estimate
- add to that the pessimistic estimate
- divide that total by six

Here is the custom function itself in living black and white!

You can determine what is called the PERT standard deviation for each activity by using the formula of ...

(Pessimistic - Optimistic) / 6.

So I created a new custom function for this and it is even more simple than the three point estimate.

Here is how it would breakdown for a pair of activities at the following proposed duration days.

You can determine what is called the PERT variance for each activity by using the formula of ...

((Pessimistic - Optimistic) / 6) ^ 2.

Here is how it would breakdown for a pair of activities at the following proposed duration days.

© 2010 - Dwayne Wright -
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