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PMP - Three Point Estimate

Written By Sambasivarao on Friday, January 18, 2013 | Friday, January 18, 2013

1. Three Point Estimate:

The 3 point estimate belongs to the time management knowledge area. It can also be used during Cost Estimation.

A three point estimate is a better estimate, compared to a single point estimate. Single point estimate simply gives you a single number – for example,
Manager: How long will it take to complete the engine design?
Engineer: 5 days.
The problem with single point estimates is that they are seldom correct.
How reliable is this 5 days estimate? It will depend on the engineer, and whether this task has been done before or not? If it is a routine task, and has been performed many times, a single point estimate may be ok…
But if it is something never done, or is a new activity, or the engineer is new to this activety, this number may well be incorrect. In such cases, going for a three point estimate will give you more reliability.
Three point estimate looks at the most optimistic estimate (O), a most likely estimate (M), and a pessimistic estimate (Least likely estimate) or (L).
In simple terms, let’s take the same question, and ask the engineer again, as follow:
Manager: Usually how long does it take to complete the engine design?
Engineer: About 5 days.
Manager: Great. And what if there were problems… unexpected issues come up, and there are new things that we haven’t thought up of arise?
Engineer: Well, in that case, it might take as long as 10 days, worst case!
Manager: I see. And how about if everything goes smoothly. No hiccups, and no problems? What’s the best case?
Engineer: Then I could get it done in just 3 days flat.
So the manager could then take a simple average, and get the three point estimate as:

Three Point Estimate = (O + M + L) / 3
E = (5 + 10 + 3) / 3 = 18/3 = 6 Days.
Or the manager could add some weightings to the estimated. Like employing PERT (Program Evaluation and Review Technique). This gives you a better estimate.
A commonly used weighting might be – there is less chance of the worst case or the best case happening. In good faith, most likely estimate (M), is what it will take to get the job done. So with this, the three point weighted average estimate would be:
Three Point Estimate = (O + 4M + L ) / 6
E = (3 + 4*5 + 10) / 6 = (3 + 20 + 10) / 6 = 33/6 = 5.6 Days.
This three point weighted average estimate is a much better technique. Use it first, if all the 3 estimates are given.
So when do you use teh Simple Average 3 point estimate? Only when the question explicitly say – using the Tringular Distribution method.
PMP Mock Exam Question on Three Point Estimate:
Q1: Moon Travels are planning on implementing a travel booking system. In the past they were using a completely manual system, which caused errors and delays for their customers. The project manager is confident that the implementation will be completed in 6 weeks, with a most optimistic estimate being 4 weeks, and the worst case scenario might take it to 11 weeks. What must be the three point estimate for this travel booking system implementation?
A: 4 weeks

B: 6 weeks
C: 11 weeks
D: None of the Above.

Doing the calculations, you can arrive at

E = ( 4 + 4*6 + 11) / 6
E = (4 + 24 + 11) / 6 = 39 / 6 = 6.5 weeks.

So the current answer would be D.
Q2: You are the project manager of Libra Travels SSS Project.  You have decided to use the 3 point estimation technique for his project. For the “Test Corporate Bookings” activity, the following estimates are available. O=4, P=9, M=5. What is the result of this estimate?
A: 5.5 weeks

B: Eighteen weeks
C: 33.33 Days
D: Three weeks

Doing the calculations, you can arrive at

E = ( 4 + 4*5 + 9) / 6
E = (4 + 20 +9) / 6 = 33 / 6 = 5.5 weeks.

So the current answer would be A.
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