BIASES OF PROBABILITY ASSESSMENT - A COMPARISON OF FREQUENCY AND SINGLE-CASE JUDGMENTS

Gigerenzer (1991) has recently argued that there is a fundamental dist inction between judgments of relative frequency and judgments of singl e-case probabilities. One of Gigerenzer's major findings is that many representativeness effects, such as the neglect of base rates and the conjunction error, disappear when questions are reworded in terms of r elative frequencies. In this paper, we claim that Gigerenzer's distinc tion between frequency and single-case judgments maps onto Tversky and Kahneman's (1974) distinction between representativeness and availabi lity. Specifically, we suggest that representativeness effects occur p rimarily in single-case judgments, while availability effects occur pr imarily in judgments of relative frequency. In Experiment 1, 170 under graduate subjects were given four ''representativeness'' problems and three ''availability'' problems. Half of the subjects made judgments o f relative frequencies, while half provided single-case probabilities. The results generally support our predictions. Representativeness eff ects tend to be stronger in the single-case condition, while availabil ity effects tend to be stronger in the frequency condition. In Experim ent 2 (N = 165), we used slight modifications of the original availabi lity problems. Again, the availability effects were more prevalent in the frequency condition than in the single-case condition. The normati ve and descriptive implications of these findings are discussed. (C) 1 995 Academic Press, Inc.