Chapter 10: Decomposition and Recomposition

Overview

Applicability of Techniques in This Chapter
	Decomposition by Feature or Task	Decomposition by Work Breakdown Structure (WBS)	Computing Best and Worst Cases from Standard Deviation
What's estimated	Size, Effort, Features	Effort	Effort, Schedule
Size of project	S M L	- M L	S M L
Development stage	Early-Late (small projects); Middle-Late (medium and large projects)	Early-Middle	Early-Late (small projects); Middle-Late (medium and large projects)
Iterative or sequential	Both	Both	Both
Accuracy possible	Medium-High	Medium	Medium

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. This chapter discusses the basic practice in more detail and explains how to avoid such pitfalls.