Availability of information and the aggregation of confidence in prior decisions

Recent research on calibration has shown that judgments about aggregate performance are consistently lower in magnitude than confidence-judgments about single items (the ''aggregation effect''). Three explanations of this effect have been proposed: Probabilistic Mental Models theory, the regression-to-the-mean hypothesis, and the dual-source hypothesis. In two studies, we tested predictions based on these explanations about the influence of availability of information on the aggregation effect. Study 1 showed that neither reducing the item set size for aggregate-item judgments nor delaying the single-item judgments eliminated the effect, Study 2 showed a persistent aggregation effect for different kinds of item lists and reminders. Further comparisons showed that discrimination (as distinguished from overconfidence) was (1) consistently better for single-item judgments than for aggregate-item judgments, and (2) improved when there is a delay between the choosing of an answer and the rating of confidence about that answer. The three proposed explanations of the aggregation effect are compared in light of these findings. (C) 1996 Academic Press, Inc.