November 5, 2020, 8:30 a.m. to 6 p.m. EDT | November 6, 2020 – February 7, 2021, On-Demand | Online Conference
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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.
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.
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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