Part VII — Presenting an interval

How do you hand an interval to somebody who asked for a number?

Two chapters. The first decides whether everything before it was worth the effort. A model nobody acts on has the same value as a model that was never built.

ch21 · A TCO for a finance audience is the twenty minutes in which the work is used or ignored: what to put in front of the person whose decision it is, why a decision table beats an interval, and the one sentence that has to go beside the number, saying what it leaves out. Everything before this chapter exists to earn the right to say it. It is also the one chapter in the book arguing from experience rather than from a stamped result, and it says so.

ch22 · Comparing two TCOs is the comparison a decision usually turns on: two quotes for the same workload, each with an interval, and a reader who cannot tell them apart. It subtracts the two totals future by future, because both designs face the same future, and the difference comes out far narrower than either total. It ends with what a comparison somebody else built has to show before you believe it.