7.4 Use Judgment Only as a Last Resort

So-called expert judgment is the least accurate means of estimation. Estimates seem to be the most accurate if they can be tied to something concrete. In the story told at the beginning of this chapter, the worst estimate was the one created by the expert who used judgment alone. Tying the estimate to the room occupancy limit was a little better, although it was subject to more error because that approach required a judgment about how full the room was as a percentage of maximum occupancy, which is an opportunity for subjectivity or bias to contaminate the estimate.

Historical data combined with computation is remarkably free from the biases that can undermine more judgment-based estimates. Avoid the temptation to tweak computed estimates to conform to your expert judgment. When I wrote the second edition of Code Complete (McConnell 2004a), I had a team that formally inspected the entire first edition—all 900 pages of it. During our first inspection meeting, our inspection rate averaged 3 minutes per page. Realizing that 3 minutes per page implied 45 hours of inspection meetings, I commented after the first meeting that I thought we were just beginning to gel as a team, and, in my judgment, we would speed up in future meetings. I suggested using a working number of 2 or 2.5 minutes per page instead of 3 minutes to plan future meetings. The project manager responded that, because we had only one meeting's worth of data, we should use that meeting's number of 3 minutes per page as a guide for planning the next few meetings. We could adjust our plans later based on different data from later meetings, if we needed to.

Nine hundred pages later, how many minutes per page do you think we averaged for the entire book? If you guessed 3 minutes per page, you're right!

Tip #34

Avoid using expert judgment to tweak an estimate that has been derived through computation. Such "expert judgment" usually degrades the estimate's accuracy.