From a formal user model to design rules
Design rules sometimes seem to contradict. We examine how a formal description of user behaviour can help explain the context when such rules are, or are not, applicable. We describe how they can be justified from a formally specified generic user model. This model was developed by formalising cognitively plausible behaviour, based on results from cognitive psychology. We examine how various classes of erroneous actions emerge from the underlying model. Our lightweight semi-formal reasoning from the user model makes predictions that could be used as the basis for further usability studies. Although the user model is very simple, a range of error patterns and design principles emerge.