Understanding conditional probabilities

• Published in: Organizational behavior and human decision processes 1987-10, Vol.40 (2), p.255-269
• Main Authors: Pollatsek, Alexander, Well, Arnold D, Konold, Clifford, Hardiman, Pamela, Cobb, George
• Format: Article
• Language: English
• Subjects: Accuracy; Biological and medical sciences; Decision theory; Errors; Fundamental and applied biological sciences. Psychology; Judgments; Probability; Psychology. Psychoanalysis. Psychiatry; Psychology. Psychophysiology; Psychometrics. Statistics. Methodology; Statistical analysis; Statistics. Mathematics
• ISSN: 0749-5978; 1095-9920
• DOI: 10.1016/0749-5978(87)90015-X

In two experiments, subjects were asked to judge whether the probability of A given B was greater than, equal to, or less than the probability of B given A for various events A and B. In addition, in Experiment 2, subjects were asked to estimate the conditional probabilities and also to calculate conditional probabilities from contingency data. For problems in which one conditional probability was objectively larger than the other, performance ranged from about 25–80% correct, depending on the nature of A and B. Changes in the wording of problems also affected performance, although less dramatically. Patterns of responses consistent with the existence of a causal bias in judging probabilities were observed with one of the wordings used but not with the other. Several features of the data suggest that a major source of error was the confusion between conditional and joint probabilities.