Presentation and Content: The Use of Base Rates as a Continuous Variable

Do subjects, in probability revision experiments, generally neglect base rates due to the use of a representativeness heuristic, or does the use of base rates depend on what we call the internal problem representation ? In Experiment 1, we used Kahneman and Tversky's (1973) engineer-lawyer problem, where random sampling of descriptions is crucial to the internal representation of the problem as one in probability revision. If random sampling was performed and observed by the subjects themselves, then their judgments conformed more to Bayesian theory than to the representativeness hypothesis. If random sampling was only verbally asserted, judgments followed the representativeness heuristic. In Experiment 2 we used the soccer problem, which has the same formal structure but which the subjects' every day experience already represents as a probability revision problem. With this change in content, subjects' judgments were indistinguishable from Bayesian performance. We conclude that by manipulating presentation and content, one can elicit either base rate neglect or base rate use, as well as points in between. This result suggests that representativeness is neither an all-purpose mental strategy nor even a tendency, but rather a function of the content and the presentation of crucial information.