Using Formal Models To Explore Display-Based Usability Issues

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 about abstract system models to complement the more traditional approach of usability evaluation by user testing.In particular, we address the question of what knowledge the user requires in order to successfully achieve a given task and the role the display plays in supplying and maintaining that knowledge. We illustrate our approach using a modelled web browser.We present two abstract models of the display and discuss how these differing designs affect user knowledge and therefore behaviour. We expose an apparently paradoxical result: that improving the user knowledge about the device may lead to more error-prone behaviour. This result leads us to a discussion of what it actually means (and does not mean) for a device to be 'usable'. (C) 1999 Academic Press.