Formality and user modelling in the design of interactive systems

In order to evaluate the usability of a proposed device the designer must make assumptions about the behaviour of the user population. This paper addresses how user assumptions can be expressed mathematically so that they can be integrated with an abstract device model. Such an integrated model allows usability decisions to be made during the system design cycle. In particular we address the question of what knowledge the user requires in order to successfully achieve a given task. The device display is crucial is supplying and maintaining knowledge that is useful to the user. We identify and investigate two types of user knowledge that the display supports: the user’s knowledge of the current underlying state of the device and the user’s knowledge of the effect of an action that changes the device state. We illustrate our approach using a modelled web browser. We present two abstract models of the display and argue how these differing designs affect user knowledge and therefore behaviour. We expose and discuss an apparently paradoxical result that improving the user knowledge about the device leads to more error prone behaviour.