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Topics: Estimating, Information Technology
Rough Order of Magnitude Estimate
Hello,

What is exact range for Rough Order of Magnitude Estimate?

Some suggests it is -50% to +50% and some others suggest it is -50% to +100%

I think as per PMI & PMBOK ® -50% to +50% is correct answer.

Thanks,
Chirag
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There is no exact range for ROM. It varies org to org.

Estimate Type / Range

Order of Magnitude -25% to +75%
Conceptual -30% to +50%
Preliminary -20% to +30%
Definitive -5% to +10%
Control -10% to +15%

Refer page 201 of PMBOK Guide 5ed
The most serious source and what I recommended to follow for each person who has to publish and work with estimations is Barry Bohem´s Cone of Uncertainty. Which is originally created for software it has been taken for all industries. The PMI has taken it too. And in my personal experience, after evaluating the numbers, it works for me.
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1 reply by Wade Harshman
Sep 13, 2016 10:48 AM
Wade Harshman
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Sergio, thank-you for your steady stream of quality answers. I have learned a lot by picking up your clues and going to Google or Amazon.
Sep 13, 2016 9:17 AM
Replying to Sergio Luis Conte
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The most serious source and what I recommended to follow for each person who has to publish and work with estimations is Barry Bohem´s Cone of Uncertainty. Which is originally created for software it has been taken for all industries. The PMI has taken it too. And in my personal experience, after evaluating the numbers, it works for me.
Sergio, thank-you for your steady stream of quality answers. I have learned a lot by picking up your clues and going to Google or Amazon.
...
1 reply by Sergio Luis Conte
Sep 13, 2016 1:05 PM
Sergio Luis Conte
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You are welcome Wade. I am here because is the best place I found to learn from others and improve myself.
I agree with Sergio and Anupam on their answers - I can't think of anything else to add other than mentioning that the degree on certainty will be closer to the actual with experience.
Sep 13, 2016 10:48 AM
Replying to Wade Harshman
...
Sergio, thank-you for your steady stream of quality answers. I have learned a lot by picking up your clues and going to Google or Amazon.
You are welcome Wade. I am here because is the best place I found to learn from others and improve myself.
Traditionally it has always been -50% to +100% based on the work of Bauman in 1958 (the actual ROM values were in use long before Bauman. He confirmed that those numbers were accurate. Barry Boehm (1981) stated that the ROM is closer to -75% to +400%.

Previous versions of the PMBOK have always used the traditional values per Bauman. The current version (5) of the PMBOK uses -25% to +75%. Somehow we've become better in guessing (which is what the ROM is). I don't know what the justification for that change was.

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