Agility Guide

A pleasing aspect (amongst many) of Kent Beck's eXtreme Programming technique is the insistence on writing test cases before writing the code. The test case defines some yardstick the implemented code has to match, and what the tester does to ensure this match. The fit criterion is slightly simpler: It is the yardstick. By adding a fit criterion to the requirement, you are, in essence, writing its test case.

Do rabbit projects need fit criteria? Absolutely, if they are to develop the right code. Rabbits do not necessarily write fit criteria as suchafter all, they may not even be writing the requirementsbut understanding fit criteria means their code solves the real problem. We suggested in Chapter 8 that rabbits use a blog to discover the nonfunctional requirements. For each of the nonfunctional requirements yielded by the blog, we now suggest deriving the appropriate fit criterion, confirming it with the stakeholder, and writing the test case using that fit criterion.

Horse projects need to have a precise understanding of the meaning of requirements. It has been our experience that when the project has multiple stakeholdersthis is the norm for horse projectsdifferent stakeholders have different meanings for requirements. Adding a fit criterion to each requirement means it is virtually impossible for misunderstandings to occur. We recommend that horse projects write fit criteria for their requirements.

Elephant projects must use fit criteria. These projects are forced to produce a written specification to be handed on to some other party, either another part of the organization or an outsourcer. Having a specification containing only unambiguous, testable requirements is crucial to elephant projects if the other party is to deliver the correct product.