Direct measures of availability and judgments of category frequency

Tversky and Kahneman (1973) have proposed that people judge the numerosity of categories by the availability of category instances. The present study tests this hypothesis using several direct measures of availability for lexicographically defined categories (e.g., “How many countries are there beginning with the letter—?”). Although these availability measures were successful in predicting some estimates, including several judgmental errors, the role of availability in frequency estimation appeared to be quite complex. A number of possible confounding factors are discussed.