Presentation and content: The use of base rates as a continuous variable.

Abstract

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. Th is 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. From its origins circa 1660 until the mid-nineteenth century, probability theory was closely identifi ed with rational thinking. In Laplace’s famous phrase, probability theory was believed to be “only common sense reduced to calculus” (Laplace, 1814/1951, p. 196). For the classical probabilists, their calculus codifi ed the intuitions of an elite of reasonable men in the face of uncertainty. And if these reasonable intuitions deviated from the laws of probability theory, it was the latter that were cast into doubt. Such discrepancies actually infl uenced the way in which probability theory developed mathematically (Daston, 1980). In the early decades of the nineteenth century, probability theory shifted from being a description of the intuitions of rational individuals to one of the behavior of the irrational masses (Porter, 1986). But in the 1960s and 1970s experimental psychology reestablished the link between probability theory and rational thinking under uncertainty. However, the new alliance diff ered from the old in two important respects. First, it was now probability theory, rather than intuitive judgments, that was the normative standard. Although probabilists have from time to time doubted whether the additivity law holds in all cases (Shafer, 1978), and although there is evidence that diff erent statistical approaches suggest diff erent answers to the same problem (Birnbaum, 1983), psychologists have generally assumed that statistics spoke with one voice—a necessary assumption for the new normative approach. Second, the link between probability theory and human thinking has become the subject of experimental research. First, by using urn-and-balls problems (e.g., Edwards, 1968; Phillips & Edwards, 1966) and then more