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Purpose. To develop an inventory to measure the influence of cognitive biases on medical decisions. Method. The Inventory of Cognitive Biases in Medicine (ICBM) consists of 22 medical scenarios in which respondents choose between alternatives that represent bias-prone or statistically based decisions. In 1992 and 1993 the ICBM was administered to 102 medical students and residents and 318 physician faculty at the Wright State University School of Medicine. Results. The statistically correct decision was made in fewer than half the ICBM scenarios for both groups (students and residents, 41%; faculty, 49%); and only the faculty scored better than chance. Acceptable reliability (KR 20 = .62 for the faculty administration) and item discrimination (mean = .49) were achieved. Further, the results suggest that the ICBM has both content and construct validity. Conclusion. The rather poor performances of the students and residents and the faculty on the ICBM suggest that cognitive biases constitute an important detraction from reliance on logical and statistical strategies. The ICBM shows promise for use as a tool in both instructional and research endeavors attempting to minimize and elucidate this phenomenon. © 1994 Association of American Medical Colleges.