A requirement that is difficult or impossible to satisfy can lead to one of two extremes: (I) an unsatisfied, impractical requirement that is too strong, or (2) a requirement that has been unnecessarily weakened or abandoned. Traditional software engineering methods are not well suited to addressing problems caused by requirements that are too weak or too strong because such methods focus on changing specifications, not requirements.

The premise of this work is that changing requirements is an important alternative to changing specifications. My thesis is that knowledge-based analysis of requirements and environmental constraints supports requirements change by allowing a program to find the strongest requirement satisfied by a specification.

This dissertation makes two contributions. First, it defines a relation called IS-STRONGER-THAN that allows a program to compare the strength of alternative requirements. A program called GIRAFFE uses a set of transformations based on the IS-STRONGER-THAN relation to incrementally strengthen and weaken requirements. Second, the dissertation describes a method for finding general scenarios. GIRAFFE uses general scenarios to detennine when requirements changes are appropriate. GIRAFFE's method for finding scenarios allows it to use partially-specified initial states and find scenarios that include multiple paths and more general object types.

GIRAFFE uses the IS-STRONGER-THAN relation and general scenarios to find the strongest requirement satisfied by a specification. The program thus helps an analyst change requirements and so avoid the problems of unsatisfied, impractical requirements and unnecessarily weak requirements.