Estimates: ROM vs Definitive for PMP Exam

In project cost and time management, estimations are often used in arriving at the cost and time at various stage along the project management cycle as the accurate cost and time will not be 100% known until the very end of the project. In estimating the cost and time, there are two important estimate metrics: Rough Order of Magnitude (ROM) Estimate vs Definitive Estimate (note: the degree of accuracy of the estimates must be clearly stated in the documents).

This post will expound on the similarities and differences of ROM Estimate vs Definitive and what Aspirants would need to know for the exam.

ROM Estimate vs Definitive Estimate

• Rough Order of Magnitude (ROM) Estimate: the differences between estimates and actual figures may be as large as 75% more or 25% less
  • used in early stage of the project when things are not very solid/clear and/or for projects that span a lengthy period
• Definitive Estimate: the differences between estimates and actual figures may be within the range of -5% to + 10% (more or less)
  • used in latter stage of the project and/or the project is simple to estimate

In addition to the ROM Estimate and Definitive Estimate, there are some more estimation ranges:

• Preliminary estimate: within the range of -15% to +50%
• Budget estimate: within the range of -10% to +25%
• Final estimate: 100% accurate

The degree of accuracy of the estimations will become more accurate as the project proceed as many unknowns at the beginning of the project will become known later on.

ROM Estimate vs Definitive Estimate Illustrated

Let’s take the project of PMP Exam study and preparation again for the illustration of the degree of accuracy of estimation.

At the very beginning of the exam preparation, the Aspirants may have little or no previous knowledge about the exam syllabus and the PMBOK® Guide. The actual period of time required for the studies may not be known accurately. The only way to estimate the study period is through the lessons learned of other exam takers.

And as a rule of thumb, the estimated preparation period for the exam is roughly 3 months of part-time study, which is considered a Rough Order of Magnitude (ROM) Estimate.

But after going through the exam prep course, the PMP Aspirant will then be able to understand the topics for the exam and their knowledge gaps. Hence a more accurate estimate can be made (e.g. 100 days — more like a Definitive Estimate). And at that time, it is often advisable to book the exact date of the exam, which can also act as a driving force for the PMP study.

Mock Exam Question

1. The project is working into 3/4 of project duration and the final component is to be built to improve the accuracy of the system. The project manager is updating the cost of the whole system. What is the degree of accuracy of the estimate?
   1. Preliminary Estimate
   2. Rough Order of Magnitude Estimate
   3. Budget Estimate
   4. Definitive Estimate
   Solution: D
   Since it is near the end of the development of the system, most of the costs of the system have already been known. The only variable is that “final component”. It could be inferred that the cost estimate for the whole system at this point should be well within -5% to +10% (i.e. Definitive Estimate)

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Conclusion: ROM Estimate vs Definitive Estimate

The various degree of accuracy of estimations mentioned in the PMBOK® Guide and other PMP Exam Prep books are listed below:

• Rough Order of Magnitude (ROM) Estimate: within the range of -25% to +75%
• Preliminary estimate: within the range of -15% to +50%
• Budget estimate: within the range of -10% to +25%
• Definitive Estimate: within the range of -5% to +10%
• Final estimate: 100% accurate

Hope this article can illustrate the differences between ROM Estimate vs Definitive Estimate well.

Additional FREE PMP resources: 47+ Commonly Confused Term Pairs with detailed explanations. If you found this article useful, you may wish to reference other Commonly Confused Term articles.