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Representativeness heuristic
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==Tversky and Kahneman's classic studies== ===Tom W.=== In a study done in 1973,{{sfn|Kahneman|Tversky|1973}} Kahneman and Tversky divided their participants into three groups: *"Base-rate group", who were given the instructions: "Consider all the first-year graduate students in the U.S. today. Please write down your best guesses about the percentage of students who are now enrolled in the following nine fields of specialization." The nine fields given were business administration, computer science, engineering, humanities and education, law, library science, medicine, physical and life sciences, and social science and social work.{{sfn|Kahneman|Tversky|1973}} *"Similarity group", who were given a personality sketch. "Tom W. is of high intelligence, although lacking in true creativity. He has a need for order and clarity, and for neat and tidy systems in which every detail finds its appropriate place. His writing is rather dull and mechanical, occasionally enlivened by somewhat corny puns and by flashes of imagination of the sci-fi type. He has a strong drive for competence. He seems to feel little sympathy for other people and does not enjoy interacting with others. Self-centered, he nonetheless has a deep moral sense." The participants in this group were asked to rank the nine areas listed in part 1 in terms of how similar Tom W. is to the prototypical graduate student of each area.{{sfn|Kahneman|Tversky|1973}} *"Prediction group", who were given the personality sketch described in 2, but were also given the information "The preceding personality sketch of Tom W. was written during Tom's senior year in high school by a psychologist, on the basis of projective tests. Tom W. is currently a graduate student. Please rank the following nine fields of graduate specialization in order of the likelihood that Tom W. is now a graduate student in each of these fields."{{sfn|Kahneman|Tversky|1973}} The judgments of likelihood were much closer for the judgments of similarity than for the estimated base rates. The findings supported the authors' predictions that people make predictions based on how representative something is (similar), rather than based on relative base rate information. For example, more than 95% of the participants said that Tom would be more likely to study computer science than education or humanities, when there were much higher base rate estimates for education and humanities than computer science.{{sfn|Kahneman|Tversky|1973}} ===The taxicab problem=== In another study done by Tversky and Kahneman, subjects were given the following problem:<ref name="tk82" /> <blockquote>A cab was involved in a hit and run accident at night. Two cab companies, the Green and the Blue, operate in the city. 85% of the cabs in the city are Green and 15% are Blue.<ref name="tk82" /> A witness identified the cab as Blue. The court tested the reliability of the witness under the same circumstances that existed on the night of the accident and concluded that the witness correctly identified each one of the two colours 80% of the time and failed 20% of the time.<ref name="tk82" /> What is the probability that the cab involved in the accident was Blue rather than Green knowing that this witness identified it as Blue?<ref name="tk82" /> </blockquote> Most subjects gave probabilities over 50%, and some gave answers over 80%. The correct answer, found using [[Bayes' theorem]], is lower than these estimates:<ref name="tk82" /> *There is a 12% probability (0.12 = 0.15 Γ 0.80) that the blue cab is (correctly) identified by the witness as blue. *There is a 17% probability (0.17 = 0.85 Γ 0.20) that the green cab is (incorrectly) identified by the witness as blue. *There is therefore a 29% probability (0.29 = 0.12 + 0.17) that the cab is identified by the witness as blue. *This results in a 41% probability (0.41 ≈ 0.12 Γ· 0.29) that the cab identified as blue was actually blue. This result can be achieved by Bayes' theorem which states: <math>P(B|I) = \frac{P(I | B)\, P(B)}{P(I)}.</math> where: P(x) - a probability of x, B - the cab was blue, I - the cab is identified by the witness as blue, B | I - the cab that is identified as blue, was blue, I | B - the cab that was blue, is identified by the witness as blue. [[Image:TaxicabProblem.png|border]] Representativeness is cited in the similar effect of the [[gambler's fallacy]], the [[regression fallacy]] and the [[conjunction fallacy]].<ref name="tk82" />
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