Chapter 1: What Is an

It is very difficult to make a vigorous, plausible, and job-risking defense of an estimate that is derived by no quantitative method, supported by little data, and certified chiefly by the hunches of the managers.
—Fred Brooks

Overview

You might think you already know what an estimate is. My goal by the end of this chapter is to convince you that an estimate is different from what most people think. A good estimate is even more different.

Here is a dictionary definition of estimate: 1. A tentative evaluation or rough calculation. 2. A preliminary calculation of the cost of a project. 3. A judgment based upon one's impressions; opinion. (Source: The American Heritage Dictionary, Second College Edition, 1985.)

Does this sound like what you are asked for when you're asked for an estimate? Are you asked for a tentative or preliminary calculation—that is, do you expect that you can change your mind later?

Probably not. When executives ask for an "estimate," they're often asking for a commitment or for a plan to meet a target. The distinctions between estimates, targets, and commitments are critical to understanding what an estimate is, what an estimate is not, and how to make your estimates better.