Accident Causation Model

The Accident Causation Model (or "Swiss Cheese Model") is a theoretical model that illustrates how accidents occur in organisations. The model focuses on both organisational hierarchy and human error. It postulates that the typical accident occurs because several (human) errors have occurred at all levels in the organisational hierarchy in a way that made such accident inavoidable. For example, decision makers may have made ill decisions when purchasing aircraft (fallible decisions), line management may have pushed for faster turnarounds (line management deficiencies), pilots may have felt pressurised to cope with a stressful climate, an unsafe culture and little rest (preconditions), the particular pilot who suffered the accident may have gotten distracted with other tasks three seconds prior to the accident (unsafe act), and the aircraft systems fail in providing unmistakable warnings of the danger (inadequate defences).

Above example illustrates key concepts in the Accident Causation Model:
  • Active errors (also called unsafe acts) are the proximal causes of the accident: the pilot got distracted. Hadn't the pilot got distracted, he would have prevented the accident.
  • Latent errors are the remaining elements in the organisation which contributed to the accident: senior managers purchasing decisions, line management pressures, unsafe climate and culture coupled with fatigue and confusing warnings. Hadn't any of these latent errors occurred, the accident would have been prevented.
  • Windows of opportunity refer to the opportunity for those active and latent errors to contribute to an accident. Hadn't the worker got distracted, he would have prevented the accident… this time. Yet, the latent errors remain unresolved, waiting for their opportunity (thus a "window of opportunity") to strike.
  • Causation chain refers to the alignment of all necessary windows of opportunity at all levels in the organisation, thus leading to the occurrence of a particular accident. That is, the causes of most accidents can be traced back to "windows of opportunity" opened at all levels in the organisation.

The Accident Causation Model was first published by Reason in 19901. Since then, it has progressively influenced contemporary views on the management of human error in organisations. For instance, the International Civil Aviation Organisation (ICAO) has formally adopted Reason's model to facilitate a systemic understanding of human factors issues within the aviation community (Wiegmann & Shappell, 20032).

(Image embedded from on 26 September 2009)
1. REASON James (1990). Human error. Cambridge University Press (UK), 1992.
2. WIEGMANN Douglas A & Scott A SHAPPELL (2003). A human error approach to aviation accident analysis: the human factors analysis and classification system. Ashgate Publishing Ltd (England, UK), 2004.

Want to know more?

AviationKnowledge - Human Factors Analysis and Classification System (HFACS)
This AviationKnoweldge page discusses the HFACS model further.
Wiki of Science - Accident Causation Model
This webpage provides additional information on Reason's Accident Causation Model including a discussion of key concepts and criticisms to the model.

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