Accident causation is illustrated with four slices of Swiss cheese threaded on an arrow. The arrow represents hazards which lead to losses, the arrow head. The four slices of cheese are described as successive layers of defenses, barrier and safeguards. The arrow passes through the holes in the slices of cheese. Some holes are due to active failures and some holes are due to latent conditions (resident "pathogens").

Figure 2

The “Swiss Cheese” Model of Accident Causation

Source: Reason J, Carthey J, deLeval M. Diagnosing “vulnerable system syndrome”: An essential prerequisite to effective risk management. Qual Health Care, 2001; 10(Suppl. II):ii21–ii25. Reprinted with permission of the BMJ Publishing Group.

From: Chapter 5, Understanding Adverse Events: A Human Factors Framework

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Patient Safety and Quality: An Evidence-Based Handbook for Nurses.
Hughes RG, editor.
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