ORGANIZATIONAL BEHAVIOR AND HUMAN DECISION PROCESSES, VOLUME 69, ISSUE 3

estimates and determine their influence on the overall In making major decisions (e.g., about medical treataggregate judgment. In the following section, I suggest ment, acceptance of manuscripts for publication, or that weighting and trimming are two important heurisinvestment), decision makers frequently poll the opintics in the aggregation of opinions under uncertainty. ions and subjective estimates of other judges. The agIn the studies that follow, I examine the use of these gregation of these opinions is often beset by difficultcognitive heuristics in studies in which individual deciies. First, decision makers often encounter conflicting sion makers were required to create aggregate estisubjective estimates. Second, estimates are often exmates on the basis of different samples of judgments. pressed with a measure of uncertainty. The decision The results from these studies are contrasted with the maker thus needs to reconcile inconsistencies among judgmental estimates and determine their influence findings of a normative study using a computer simulaon the overall aggregate judgment. In the empirical tion that was designed to assess the objective effects of studies, I examine the idea that weighting and trimweighting and trimming operations on the accuracy ming are two important heuristics in the aggregation of estimation. of opinions under uncertainty. The results from these studies are contrasted with the findings of a normative AGGREGATION PROCESSES study using a computer simulation that was designed to assess the objective effects of weighting and trimIn the following section, I review the cognitive psychoming operations on the accuracy of estimation. q 1997 logical bases for weighting and trimming processes in Academic Press judgmental aggregation of estimates. In parallel, I also consider the normative conditions under which one It is common practice to poll the opinions and judgmight expect weighting and trimming operations to inments of knowledgeable individuals before making macrease accuracy over simple averaging. jor decisions. Patients often seek several opinions before Previous research has considered simple averaging deciding on radical surgery, faculty deans poll opinions as a psychological model of the aggregation process (Anabout candidates, journal editors consult referees conderson, 1981; Dawes, 1979; Einhorn & Hogarth, 1975; cerning publication decisions, and business managers Einhorn, Hogarth, & Klempner, 1977; Hastie, 1986; seek expert forecasts before embarking on risky projSniezek & Henry, 1989). Numerous empirical studies on ects. forecasting and estimation have also suggested simple The research presented here concerns two important averaging of individual opinions as a normative scheme issues that affect the aggregation process. First, deciand a method for improving the accuracy of predictions sion makers often encounter conflicting opinions and (Armstrong, 1985; Ashton, 1986; Hill, 1982; Hogarth, judgments. Second, judgments are often expressed with 1978; Zajonc, 1962; Zarnowitz, 1984). Due to its promia measure of uncertainty. The decision maker thus nence in the judgment literature, simple averaging is needs to reconcile inconsistencies among judgmental used as a baseline in evaluating the distinctive effects of weighting and trimming operations. This research was supported by grants from the Israel Science Foundation and the Israel Foundations Trustees. I am grateful to Cognitive and Normative Bases of Weighting and Dean Foster for numerous productive discussions on this work and Trimming to Josh Klayman and anonymous reviewers for helpful comments on the manuscript. In this study, weighting was evaluated in two conAddress correspondence and reprint requests to Ilan Yaniv, Departtexts. First, I examined the use of weighting as a cogniment of Psychology, Hebrew University, Mt. Scopus, Jerusalem, Israel. Fax: 9722-588-1159. E-mail: msilan@pluto.mscc. huji.ac. il. tive heuristic in aggregation. Second, I examined its