Rough Order of magnitude

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oberoi.amit
Contributor
Posts: 125
Joined: Mon Mar 24, 2014 5:14 am

Rough Order of magnitude

Is Rough Order of Estimate and Order of Estimate are the same terms.
If yes, Can you please advise what is the correct range of Rough order of estimate and Definitive Estimate, Because it is mentioned in PMBK..
ROM: -25% to +75%
Definitive Estimate : -5% to + 10%

However if you see Andy Crowe it is mentioned:

ROM: -50% to +100%
Definitive Estimate : -15% to + 20%

saket
Posts: 3113
Joined: Sat Dec 07, 2013 10:37 pm

Re: Rough Order of magnitude

I know many practice tests play on this but i would recommend not to look at as one number to remember... lets look at the PMBOK text

The accuracy of a project estimate will increase as the project progresses through the project life cycle. For example, a project in the initiation phase may have a rough order of magnitude (ROM) estimate in the range of −25% to +75%. Later in the project, as more information is known, definitive estimates could narrow the range of accuracy to -5% to +10%. In some organizations, there are guidelines for when such refinements can be made and the degree of confidence or accuracy that is expected.

This is something we should worry about
The accuracy of a project estimate will increase as the project progresses through the project life cycle

Not about the example number used.

In reality every organization has their own standards for it, i do not expect exam will ask what is the % range of definitive estimate... the exam will ask when can we get definitive estimate and how accurate they are compare to other (ROM)
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oberoi.amit
Contributor
Posts: 125
Joined: Mon Mar 24, 2014 5:14 am

Re: Rough Order of magnitude

Thank saket, However I have seen in the practice test sessions where we need to calculate ROM, My worry part is which % we should calculate them on..
Like in the below Question by your online practice test

You are project manager in planning a road construction project. It has been decided to outsource work for digging roads. In most probability you will be using services of vendor A which will cost around 15Mn - however it may also happen that you may end up using services of vendor B which will cost you around 20Mn but as project manager you would want to use services of vendor C which will cost you 10 Mn. Your sponsor wants you to estimate cost of digging road using triangular distribution and provide range using rough order of magnitude. The range you should provide ____

a) 10 Mn – 15 Mn
b) 11.25 Mn – 26.25 Mn
c) 12.5 Mn – 18.5 Mn
d) 9.5 Mn – 20.75 Mn
saket
Posts: 3113
Joined: Sat Dec 07, 2013 10:37 pm

Re: Rough Order of magnitude

if such questions comes we need to follow elimination... like something which is in range of 10 to 20 % +/- can be first removed... and after that pick something which is near to +/- 50% variance . I see very less probability of such questions with very close options coming since we do not have defined benchmark.

The question below is not related to ROM, it is related to 3 point estimate with range... and using triangular distribution.
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kripakrish
Beginner
Posts: 41
Joined: Wed Apr 09, 2014 2:59 pm

Re: Rough Order of magnitude

so what is the anwer here?
saket
Posts: 3113
Joined: Sat Dec 07, 2013 10:37 pm

Re: Rough Order of magnitude

We have bit confusion in this question, since the question talks about triangular distribution first and then moves to ROM estimate which is not having defined limits.

The PERT Cost estimate for this would be

Most Likely = 15 Mn
Pessimistic = 20 Mn
Optimistic = 10 Mn

(15+20+10) / 3 = 15 Mn

Now we should use sigma levels to know the confidence, like the question could ask what is the range of finishing this project with 99. 7% confidence ( 3 Sigma)

we need to review this question , since it is mixing two things.
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