Decomposition and Recomposition

Decomposition

Decomposition is the practice of separating an estimate into multiple pieces, estimating each piece individually, and then recombining the individual estimates into an aggregate estimate.

This estimation approach is also known as "bottom up," "micro estimation," "module build up," "by engineering procedure," and by many other names (Tockey 2005). Decomposition is a cornerstone estimation practice—as long as you watch out for a few pitfalls.

The Law of Large Numbers

The team's estimate benefited from a statistical property called the Law of Large Numbers. The gist of this law is that if you create one big estimate, the estimate's error tendency will be completely on the high side or completely on the low side. But if you create several smaller estimates, some of the estimation errors will be on the high side, and some will be on the low side. The errors will tend to cancel each other out to some degree. Your team underestimated in some cases, but it also overestimated in some cases, so the error in the aggregate estimate is only 7%. In your estimate, all 24% of the error was on the same side.

Recomposition

Feature

Best Case (25% Likely)

Most Likely Case

Worst Case (75% Likely)

Expected Case (50% Likely)