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The Etiology of Diagnostic Errors: A Controlled Trial of System 1 Versus System 2 Reasoning

Norman, Geoffrey PhD; Sherbino, Jonathan MD; Dore, Kelly PhD; Wood, Tim PhD; Young, Meredith PhD; Gaissmaier, Wolfgang PhD; Kreuger, Sharyn; Monteiro, Sandra MSc

doi: 10.1097/ACM.0000000000000105
Research Reports

Purpose 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.

Method 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.

Results 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.

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

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.

Editor’s Note: A commentary on this article by P. Croskerry, D.A. Petrie, J.B. Reilly, and G. Tait appears on pages 197–200.

Funding/Support: The study was supported by grants from the Medical Council of Canada.

Other disclosures: None.

Ethical approval: The study received approval from the medical ethics boards at McGill University Faculty of Medicine, Montreal; McMaster University Faculty of Health Sciences, Hamilton; the University of Ottawa Faculty of Medicine, Ottawa; and the Medical Council of Canada.

Previous presentations. The study was presented as an oral abstract at the Canadian Conference on Medical Education annual meeting, Quebec City, April 2013.

Correspondence should be addressed to Dr. Norman, MDCL 3519, McMaster University, 1280 Main St. W., Hamilton, Ontario, Canada L8S 4K1; e-mail:

© 2014 by the Association of American Medical Colleges