Confidence and varieties of bias

We test the proposition that response bias can have two different bases; reflecting either differing beliefs about the a priori likelihood of competing response alternatives, or their relative utilities. In evidence accumulation models, these two types of bias are thought to manifest as variations in the starting point for accumulation and threshold for responding, respectively. Although these two mechanisms are indistinguishable for linear accumulators in terms of accuracy and RT, Vickers’ (1979) balance-of-evidence hypothesis predicts they have dissociable effects on confidence. We derived ten ordinal predictions from these models and confirmed them at the level of group averages using traditional ANOVA analyses of results from a new experiment that manipulated the probability of correct responses and the rewards associated with them. However, individual effects were more variable, particularly with respect to the reward manipulation. We then used Bamber’s (1979) state–trace analysis to test the predicted dissociations using Bayes factors developed by Prince, Brown and Heathcote (2012) and Davis-Stober, Morey and Heathcote (2016). Once again, we found support at the aggregate level but more equivocal results for individuals. We discuss why individual results are to be preferred over aggregate results in state–trace analysis and draw the lesson that tailored designs are needed to obtain clear results from individual state–trace analyses.