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The bad apple theory won't work: response to ‘Challenging the systems approach: why adverse event rates are not improving’ by Dr Levitt
  1. Sidney W A Dekker1,
  2. Nancy G Leveson2
  1. 1Safety Science Innovation Lab, Griffith University, HUM, Nathan Campus, Queensland, Australia
  2. 2Aeronautics and Astronautics and Engineering Systems Division, Massachusetts Institute of Technology, Aeronautics and Astronautics, Cambridge, Massachusetts, USA
  1. Correspondence to Professor Sidney W A Dekker, Safety Science Innovation Lab, Griffith University, HUM, N16 Macrossan Building, 170 Kessels Road, Nathan Campus, QLD 4111, Australia; s.dekker{at}

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There is no doubt about Dr Levitt's genuine concern for patient safety.1 His experience, like that of others, must indeed have led him to hospital staff he'd rather do without. One can understand the seduction of sanctioning non-compliant doctors2 or getting rid of the deficient practitioners—the system's bad apples—altogether,3 as also proposed by Levitt. In 1925, German and British psychologists were convinced they had cracked the safety problem in exactly this way. Their statistical analysis of five decades had led them to accident-prone workers; misfits whose personal characteristics predisposed them to making errors and having accidents.4 Their data told the same stories flagged by Levitt: if only a small percentage of people is responsible for a large percentage of accidents, then removing those bad apples will make the system drastically safer.

It didn't work. The reason was a major statistical flaw in the argument. For the accident-prone thesis (or bad apple theory) to work, the probability of error and accident must be equal across every worker or doctor. Of course it …

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  • Contributors Each of the two coauthors contributed an equal share of the thoughts, writing and ideas expressed in this paper.

  • Competing interests None.

  • Provenance and peer review Not commissioned; internally peer reviewed.

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