Chapter 3
Decomposition for Estimation

As we’ve seen, most approaches to estimation involve breaking the work down into smaller chunks and estimating the chunks. Not only does this make it easier to get a chunk of work for estimating completely in your head at one time, but it’s more flexible when you change plans and want to replace some planned work with something else.

Not all approaches to decomposition are the same, however. Which way should we split it? How small should the pieces be? Should they all be the same size? What do we do if we don’t know how to make them the same size? These are some of the issues we’ll explore in this chapter. In addition, there are some traps for the unwary, which we’ll explore in ​Decomposition Gotchas​.

The way you approach decomposition can affect not only your estimates, but also your ability to validate your estimates. Decomposition for estimation can also bleed through to the work itself; it’s likely you’ll use the same decomposition you used for estimating when planning the work. And if you’ve put more effort into planning than the current needs warrant, you’ll find it more difficult to change those plans when it becomes apparent they’re obsolete.

Let’s look at some different ways to decompose the anticipated work for estimation. After you’ve decided how to divide the work, then you can consider how much to do so.