Critical parameters are quantitative measures of performance that may be used to determine the overall ability of a design to serve its purpose. Although critical parameters figure in almost every field of design where there is a demand for progressive improvement, they do not appear to figure sig-nificantly in the design of interactive systems. As a result, systems are designed that are recognizably different from other systems but not necessarily better at doing the job in-tended. This paper discusses the role of critical parameters in design, and illustrates their lack of use in interactive sys-tem design by presenting a number of of examples drawn from the HCI literature. It identifies a consequent need for research to establish critical parameters for applications and to build models of the performance of designs against these parameters. Some ideas are presented on how critical pa-rameters might be established for specific applications, and the paper concludes by summarising some of the benefits that might be gained from moving in this direction.

