Options and Criteria: Elements of design space analysis

Design Space Analysis is an approach to representing design rationale. It uses a semiformal notation, called QOC (Questions, Options, and Criteria), to represent the design space around an artifact. The main constituents of QOC are Questions identifying key design issues, Options providing possible answers to the Questions, and Criteria for assessing and comparing the Options. Design Space Analysis also takes account of justifications for the design (and possible alternative designs) that reflect considerations such as consistency, models and analogies, and relevant data and theory. A Design Space Analysis does not produce a record of the design process but is instead a coproduct of design and has to be constructed alongside the artifact itself. Our work is motivated by the notion that a Design Space Analysis will repay the investment in its creation by supporting both the original process of design and subsequent work on redesign and reuse by (a) providing an explicit representation to aid reasoning about the design and about the consequences of changes to it and (b) serving as a vehicle for communication, for example, among members of the design team or among the original designers and later maintainers of a system. Our work to date emphasises the nature of the QOC representation over processes for creating it, so these claims serve as goals rather than objectives we have achieved. This article describes the elements of Design Space Analysis and illustrates them by reference to analyses of existing designs and to studies of the concepts and arguments used by designers during design discussions.