SPECIFYING USER KNOWLEDGE FOR THE DESIGN OF INTERACTIVE SYSTEMS

Minimum requirements for interactive systems to be usable and reliable include computer systems performing as intended, and users not making errors in issuing commands or in interpreting information from the de vice display, Traditionally, most approaches to software engineering h ave focused on the first of these concerns; correctness of system perf ormance. However, it is equally important to deal with the user concer ns. An Instruction Language is presented for describing the knowledge a user needs to perform tasks with the device. The constraints provide d by a semi-formal description language help the designer to identify possible mismatches between the system model and the user's model of t hat system. This type of mismatch is illustrated with an example taken from the design of the Macintosh desktop. If a further step is taken, formalising that description and adding principles about users' cogni tive processes, inferences may also be made about possible user errors . This is illustrated with an example taken from the design of a mail tool. The Instruction Language and associated principles provide a mea ns of evaluating system design in relation to user knowledge prior to implementation.