Processes Mediating Judgements of Likelihood

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Likelihood judgment plays a critical role in decisions that are made inalmost all of life's domains. Often, people must judge the likelihood ofone event when more than one alternative event is known to be possible(e.g., the likelihood that a patient has condition A rather than conditionB, C, or D; the likelihood that a contract for a construction project willbe awarded to bidder F rather than G or H; the likelihood that a friend isarriving on airline K rather than M or L). The proposed research willexamine fundamental processes underlying such likelihood judgments. Recentresearch by the primary investigator suggests that judgments of likelihoodare often mediated by a distinct comparison heuristic; such judgments arelargely dependent upon a comparison between the strength of the evidencefor the focal event and the strongest alternative event, rather than aweighing of the evidence for the focal event versus the aggregate of theevidence for all alternative events. The comparison heuristic is veryefficient and generally produces reasonable judgments of likelihood.However, the heuristic can also underlie patterns of perceived likelihoodthat are not consistent with the normative dictates of standard probabilitytheory. Nine experiments will investigate the nature, strength, and scopeof the comparison heuristic. They will also examine how task conditionsthat vary under everyday judgment situations--time pressure, the type ofresponse being solicited, and the quantifiability of the availableevidence--influence likelihood judgments. The experiments will involve awide range of paradigms and judgment types. Various methodological tools(e.g., manipulations to the distributions of alternative outcomes, processtracing) will allow for an assessment of the relative influence ofdeliberative versus heuristic processes within manipulated conditions ofthe experiments. This research should substantially advance theunderstanding of processes mediating judgments of likelihood as well asenhance researchers' abilities to predict judgments (and decisions andbehaviors) that are based on perceptions of focal events that have multiplealternatives.