Decomposition by Feature or Task

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.

Software development is a process of making larger numbers of steadily smaller decisions. At the beginning of the project, you make such decisions as "What major areas should this software contain?" A simple decision to include or exclude an area can significantly swing total project effort and schedule in one direction or another. As you approach top-level requirements, you make a larger number of decisions about which features should be in or out, but each of those decisions on average exerts a smaller impact on the overall project outcome. As you approach detailed requirements, you typically make hundreds of decisions, some with larger implications and some with smaller implications, but on average the impact of these decisions is far smaller than the impact of the decisions made earlier in the project.

By the time you focus on software construction, the granularity of the decisions you make is tiny: "How should I design this class interface? How should I name this variable? How should I structure this loop?" And so on. These decisions are still important, but the effect of any single decision tends to be localized compared with the big decisions that were made at the initial, software-concept level.

The implication of software development being a process of steady refinement is that the further into the project you are, the finer-grained your decomposed estimates can be. Early in the project, you might base a bottom-up estimate on feature areas. Later, you might base the estimate on marketing requirements. Still later, you might use detailed requirements or engineering requirements. In the project's endgame, you might use developer and tester task-based estimates.