Ambiguous Specifications

The specification should, as far as is practical, be free of ambiguity. You should not have used any pronouns and should be wary of unqualified adjectives and adverbsall of these introduce ambiguity. Do not use the word "should" when writing your requirements; it infers that the requirement is optional. But even if you follow these guidelines, some problems may remain.

The fit criteria are devices to quantify each of the requirements, and thus make them unambiguous. We describe fit criteria in Chapter 9, explaining how they make each requirement both measurable and testable. If you have correctly applied fit criteria, then the requirements in your specification will be unambiguous.

This leaves the descriptions of the requirements. Obviously, the less ambiguity they contain, the better, but a poor description cannot do too much damage if you have a properly quantified fit criterion. However, if you are concerned about this issue, then we suggest that you select 50 requirements randomly. Take one of them and ask a panel of stakeholders to give their interpretation of the requirement. If everyone agrees on the meaning of the requirement, then set it aside. If the meaning of the requirement is disputed, then select five more requirements. Repeat this review until either it becomes clear that the specification is acceptable or the collection of selected requirements to test has grown so large (each ambiguity brings in five new ones) that the problem is obvious to all.

If the problem is bad, then consider rewriting the specification using a qualified technical writer.

The terms used in the requirements must be those defined in the Naming Conventions and Definitions section of the specification. If every word has an agreed-upon definition and you have used the terms consistently, then the meanings throughout the specification must be consistent and unambiguous.