Rough order of Magnitude vs. Definitive estimate

What is the difference between these types of estimates? When is the best time for one to be used?
Rough Order of Magnitude – What does it mean and when can I use it?
When the project is still in its early stages, the ROM method of estimation is used. You are asked to give a quick estimate. Because you don’t have the time to go through your estimates in depth, you might end up with a low level of accuracy. This could mean that your estimate could be inaccurate by as much as 25% to +75%!
How do I do it?
There are two ways to produce a ROM for your project. The first is historical, based on past experience with similar projects. Are you familiar with similar projects? What were the results, duration, cost, and lessons learned from similar projects?
Second, what is the scope or high-level of the work required and the resources that may be required?
Definitive estimates – What is it and how do I use it?
If your stakeholders require more detail or accuracy, you will need to provide precise estimates. You have all the information and the time to create a solid estimate.
Usually, you are farther along in your project planning and have more information about the scope of work, costs, and other details than an ROM. Your accuracy will be higher, anywhere from -5 to +10 percent.
Here’s a quick comparison that may help you remember which method.
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