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Acad Med. 2014 Feb;89(2):277-84. doi: 10.1097/ACM.0000000000000105.

The etiology of diagnostic errors: a controlled trial of system 1 versus system 2 reasoning.

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Dr. Norman is professor, Department of Clinical Epidemiology and Biostatistics, McMaster University, Ontario, Canada. Dr. Sherbino is associate professor, Department of Medicine, McMaster University, Ontario, Canada. Dr. Dore is assistant professor, Department of Medicine, McMaster University, Ontario, Canada. Dr. Wood is associate professor, Department of Medicine, University of Ottawa, Ontario, Canada. Dr. Young is assistant professor, Department of Medicine, McGill University, Quebec, Canada. Dr. Gaissmaier is chief research scientist, Harding Center for Risk Literacy, Max Plank Institute for Human Development, Berlin, Germany. Ms. Kreuger is research coordinator, Program for Educational Research and Development, McMaster University, Ontario, Canada. Ms. Monteiro is a PhD candidate, Department of Psychology, Neuroscience and Behaviour, McMaster University, Ontario, Canada.



Diagnostic errors are thought to arise from cognitive biases associated with System 1 reasoning, which is rapid and unconscious. The primary hypothesis of this study was that the instruction to be slow and thorough will have no advantage in diagnostic accuracy over the instruction to proceed rapidly.


Participants were second-year residents who volunteered after they had taken the Medical Council of Canada (MCC) Qualifying Examination Part II. Participants were tested at three Canadian medical schools (McMaster, Ottawa, and McGill) in 2010 (n = 96) and 2011 (n = 108). The intervention consisted of 20 computer-based internal medicine cases, with instructions either (1) to be as quick as possible but not make mistakes (the Speed cohort, 2010), or (2) to be careful, thorough, and reflective (the Reflect cohort, 2011). The authors examined accuracy scores on the 20 cases, time taken to diagnose cases, and MCC examination performance.


Overall accuracy in the Speed condition was 44.5%, and in the Reflect condition was 45.0%; this was not significant. The Speed cohort took an average of 69 seconds per case versus 89 seconds for the Reflect cohort (P < .001). In both cohorts, cases diagnosed incorrectly took an average of 17 seconds longer than cases diagnosed correctly. Diagnostic accuracy was moderately correlated with performance on both written and problem-solving components of the MCC licensure examination and inversely correlated with time.


The study demonstrates that simply encouraging slowing down and increasing attention to analytical thinking is insufficient to increase diagnostic accuracy.

[Indexed for MEDLINE]
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