Fit Criteria for Functional Requirements

A functional requirement is something that the product must doan action it must take. The fit criterion specifies how you will know that the product has successfully carried out that action. For functional requirements, there are no scales of measurement: The action is either completed or not completed. Completion depends on satisfying an authority that the product has correctly performed the action. The authority in this case is either the source of the data or the adjacent system that initiated the action.

Functional requirements can be written for different kinds of action. For example, if the action is to record something, then the fit criterion is that the recorded data complies with the data as known to the authority. For example:

Description: The product shall record the weather station readings.

Rationale: The readings are necessary for preparing the de-icing schedule.

Fit Criterion: The recorded weather station readings shall be identical to the readings as recorded by the transmitting weather station.

The authority in this case is the weather station: It initiated the action, and it is the source of the data. You can say the requirement is for the product to faithfully store the data (allow for the product to make necessary manipulations to the data) as sent from the weather station. If this is done correctly, then the product's data conforms to that transmitted by the station.

The fit criterion does not indicate how this conformance is to be tested. Instead, it is simply a statement the tester uses to ensure compliance.

If the functional requirement is to make some calculation, then the fit criterion says the result of the calculation must be consistent with the authority's view of the data. For example, if the requirement is that "the product shall record . . . ," then the fit criterion reads "the retrieved data shall agree with . . ." and cites the authority for the data. Usually this authority is the original source of the data. Where the requirement is "the product shall check . . . ," the fit criterion is "the checked data conforms with . . ." and again cites the authority for the data. "The product shall calculate . . ." results in a fit criterion of "the result conforms to . . ." and gives the algorithm (or source of the algorithm) for the result.

For the fit criteria to be complete, the specification must contain a definition of the terms used in the fit criteria.

The general rule for functional requirements is that the fit criterion ensures that the function has been successfully carried out. That brings us to test cases.

Test Cases

You might find it feasible at this stage to think about writing test cases for your functional requirements. This approach is in line with the eXtreme Programming view, which advocates writing the test before writing the code. The basic idea is to force the programmer to concentrate on learning the success criterion for any part of the functionality.

You might find it feasible to write test cases for your functional requirements.

Many requirements analysts feel uncomfortable delving into writing test cases. However, the testers for your organization will bewe are sure of thisdelighted either to help you or to write the test cases for you. Testing is most effective early in the development cycle. Involving the testers in the requirements activity is always beneficial, because testers are the best people to tell you if your functional requirements are correctly (and testably) stated.

Myers, Glenford, Corey Sandler et al. The Art of Software Testing (second edition). John Wiley & Sons, 2004.