Decision making and categorisation are the domains of interest in Chapter 6. The chapter has a subtext, however, comparative modelling. Three competing models of two medical diagnosis tasks are developed (based on Bayesian, associationist, and hypothesis testing approaches). Each of the models contains parameters that allow aspects of their behaviour to be tuned to fit empirical findings. It is suggested that an appropriate methodology for parameter estimation is to consider multiple dependent measures, and set parameters to fit only a subset of these, allowing other dependent measures to be predicted from the models. Although no single diagnosis model is found to yield a perfect account of the observed human behaviour, development of a range of models for the same tasks clarifies the strengths and weaknesses of the various approaches.