BASE-RATE EFFECTS IN MULTIDIMENSIONAL PERCEPTUAL CATEGORIZATION

The optimality of human performance when category base rates differ wa s investigated in 2 multidimensional perceptual categorization tasks. All participants were sensitive to differences in base rate, even duri ng their 1st experimental session. Nearly half of the participants lea rned the optimal decision bound by their final experimental session. L ittle evidence for conservative cutoff placement was found (i.e., an u nderestimation of category base-rate differences). In fact, participan ts who did not learn the optimal decision bound tended to use a decisi on bound that overstimated the based-rate difference. Across all condi tions participants showed a clear shift toward the optimal decision bo und with experience. These data suggest that experienced participants are highly sensitive to differences in category base rate. The model-b ased analyses suggest that the decision-bound model of categorization (Ashby, 1992a; Ashby & Maddox, 1993; Maddox & Ashby, 1993) provides a powerful tool for investigating the limits of human categorization per formance.