Key points are not available for this paper at this time.
Michael H. BirnbaumUniversity of Illinois at Urbana-ChampaignBarbara A. MellersUniversity of California, BerkeleySubjects made judgments of the probability of an event given informationand the opinion of a source. Base rate and the source's hit and false-alarm rateswere manipulated in a within-subjects design. Hit rate and false-alarm rate weremanipulated to produce sources of varied expertise and bias. The base rate, thesource's opinion, and the source's expertise and bias all had large systematic effects.Although there was no evidence of a base-rate fallacy, neither Bayes' theoremnor a subjective Bayesian model that allows for conservatism due to misperceptionor response bias could account for the data. Responses were consistent with a scale-adjustment averaging model developed by Birnbaum Peterson Slovic & Lichtenstein, 1971).Edwards (1968) discussed three interpre-tations of conservatism: misperception, mis-aggregation, and response bias. Misperceptionincludes the possibility that objective proba-bilities are transformed to subjective proba-bilities by a psychophysical function. Misag-gregation refers to use of a non-Bayesian ruleto combine evidence. Response bias refers tononlinearity in the judgment function relatingjudged probabilities to subjective likelihoods.Early experimental work attempted to separate
Birnbaum et al. (Sat,) studied this question.