Use Cases and Fit Criteria

A use case, whether it is a product use case or a business use case, is a set of actions triggered by a business event. You can think of a use case as a collection of requirementsboth functional and nonfunctionalworking toward a common end. Each requirement has its own fit criterion, which measures the requirement. If we can measure individual requirements, we can also measure the collection. In other words, we can apply a fit criterion to a use case.

You can apply a fit criterion to a use case.

To avoid confusion about this use case criterion, we call it the outcome. That is, this criterion is the intended outcome of the (business or product) use case if all works as intended. As we are talking about a collection of requirements, each with a fit criterion, you may prefer to think of the outcome as a summary (or even summation) of all the individual fit criteria.

The outcome does not replace the individual requirements' fit criteria. It does, however, help you communicate about the intention of the use case.

We suggest you make use of outcomes very early in your requirements gathering. During the blastoff (or as soon as you identify the business events), try to elicit from your stakeholders the intended outcome for each of their business events. You are asking, "When this business event happens, what does the business need to achieve?" The answer to this question is, with a little massaging on your part, the outcome or fit criterion for the business use case. As your business use cases evolve into product use cases, the same or very similar criteria can be applied. You will find, as we have, that an early outcome criterion eliminates a lot of misunderstandings about what each business use case is intended to accomplish.

As you capture individual requirements, and their fit criteria, keep in mind that each of them has to contribute in some way to the purpose of the product. If you have an outcome criterion attached to each use case, it is easier to ensure that all the requirements you are capturing contribute to the use case as a whole.