Availability: A Heuristic for Judging Frequency and Probability

Conducted a series of experiments with 877 Ss to explore a judgmental heuristic in which S evaluates the frequency of classes or the probability of events by availability (i.e., by the ease with which relevant instances comes to mind). In general, availability was correlated with ecological frequency, but it was also affected by other factors. Consequently, the reliance on the availability heuristic led to systematic biases. Such biases were demonstrated in the judged frequency of classes of words, of combinatorial outcomes, and of repeated events. The phenomenon of illusory correlation is explained as an availability bias. The effects of the availability of incidents and scenarios on subjective probability are discussed. (30 ref)