There are many different names for estimates: Rough, Ball Park, SWAG, Big Picture, Budgetary and Final to name a few. Each name attempts to convey the accuracy of the figure. All of them fall on the continuum between three points: Rough Order of Magnitude, Budgetary and Definitive.
The accuracy of the Rough Order of Magnitude (ROM) Estimate ranges from -25% to +75% off. That means a $100,000 estimate can be as low as $75,000 or as high as $175,000. It is the type of estimate usually given when very little is known about the project but management needs to decide whether to move forward or not.
A Budgetary Estimate is put together at the time the Project Charter or Statement of Work is defined. At this point enough is know about what is in and out of scope that boundaries can be established. The accuracy range is between -10% and +25% ($90K and $125K for our $100K project).
The Definitive Estimate range is between -5% and +10% ($95K and $110). This estimate is done based on detailed project information that, in IT, comes after the Requirements or Design phases.
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