Swiss cheese model

The Swiss cheese model of accident causation, developed by psychologist James Reason, posits that catastrophic failures occur when multiple defensive barriers — each with its own 'holes' or weaknesses — align, allowing a hazard to pass through every layer undetected. Each slice of cheese represents a defensive layer: engineering controls, administrative procedures, supervision, and organizational culture. The holes are latent conditions — design flaws, training gaps, resource constraints — created by organizational decisions long before the accident occurs.

The model's power is visual and pedagogical: it makes intuitive the idea that accidents are not caused by single failures but by the conjunction of multiple small failures. But the model also has limitations. It is a static, linear representation of a dynamic, nonlinear process. It does not capture feedback loops, temporal dynamics, or the emergent interactions that normal accidents theory identifies as the true source of system failure. The Swiss cheese model is a good introduction to layered defense; it is not a sufficient theory of complex system failure.