Abstract :
Formal methods applied to user interface design are important, but they may seem minor compared to the `real task´ of designing a system that suggests and supports appropriate, task-relevant actions. This is a misconception: the real task of design includes more than just making a system suggest and support task-relevant actions (and so forth). It must do so reliably, completely, and consistently: in short, it must be trustworthy. Without a formal basis, interactive systems merely give the temporary semblance of being easy to use. The author proposes an approach that is operational, and can be applied to working systems and to abstract design schemes. It is therefore applicable throughout the design cycle, at early design stages or late, indeed it can be used for conformance testing. Results are conservative and do not rely on psychological assumptions. By analysing an actual digital clock it was found that an error-blocking model retaining the same buttons would be, in one sense, about a third easier to use; he also examined an alternative design with just two buttons
