Informal Conceptions of Probability

A model of informal reasoning under conditions of uncertainty, the outcome approach, was developed to account for the nonnormative responses of a subset of 16 undergraduates who were interviewed. For individuals who reason according to the outcome approach, the goal in questions of uncertainty is to predict the outcome of an individual trial. Their predictions take the form of yes-no decisions on whether an outcome will occur on a particular trial. These predictions are then evaluated as having been either right or wrong. Their predictions are often based on a deterministic model of the situation. In follow-up interviews using a different set of problems, responses of outcome-oriented participants were predicted. In one problem, their responses were at variance both with normative interpretations of probability and with the "representativeness heuristic" (Kahneman & Tversky, 1972). Although the outcome approach is inconsistent with formal theories of probability, its components are logically consistent and reasonable in the context of everyday decision making.