There's always one!

Abstract

Informal analysis of many usability tests suggests that there is regularly one participant that is substantially slower than all the others. Moreover, such outliers are more extreme and more frequent than would be predicted by a normal distribution. We propose using a rational model to explain the outliers and the work described here begins to parameterise the model based on empirical data to provide accurate analyses of user performance. This prediction appears to be correct and the model begins to reflect the outlying performance. Moreover, by using an executable model, we believe that it could be used in future as an analytical tool to help designers improve usability for those users who are struggling the most.