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base-rate fallacy

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A failure to take account of the base rate or prior probability (1) of an event when subjectively judging its conditional probability. A classic experiment in 1973 by the Israeli psychologists Daniel Kahneman (born 1934) and Amos Tversky (1937–96) showed that people's judgements as to whether a student who was described in a personality sketch was more likely to be a student of engineering or of law tended to ignore the additional information that the student was drawn randomly from a group of 70 engineering students and 30 law students or from a group of 30 engineering students and 70 law students, although that information was highly relevant to the question in hand. The fallacy is usually explained by the use in problems of this kind of the representativeness heuristic, which is insensitive to prior probabilities. See also attributional bias, Bayesian inference, taxicab problem, Wells effect. Compare conjunction fallacy, sample size fallacy.

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