Swiss Cheese Model
Definition
James Reason's accident causation model representing each defensive barrier as a cheese slice with holes. An accident occurs when holes in successive slices momentarily align, allowing a hazard trajectory to pass all defences.
- Author
- James Reason
- Field
- Accident causation / systems safety
- Core unit
- Defensive barrier (a 'slice') with variable holes
- Failure condition
- Holes in successive slices align momentarily
- Applied in
- Aviation, healthcare, industrial safety investigations
Common questions
What distinguishes the holes in each slice from each other?+
Reason separates them into latent conditions, weaknesses built into the system by design, training, or maintenance decisions, and active failures, the immediate unsafe acts of an operator. Latent holes can exist for a long time before an active failure aligns with them.
How is the model used in an actual incident investigation?+
Investigators map each barrier that should have stopped the hazard, whether it was a procedure, an interlock, supervision, or training, and identify why each one failed at that moment, producing a causal chain rather than blaming a single person.
Is the Swiss Cheese Model considered a complete explanation of accident causation today?+
It remains widely taught as an introductory framework, but safety researchers now often pair it with systems-based models such as STAMP or FRAM, which better capture dynamic interactions between components rather than static, independent barriers.
Related terms
- Active Failure
- An error or rule violation by a front-line operator whose effects are felt immediately. Active failures are the triggers Reason calls 'unsafe...
- Blowout Preventer (BOP)
- A large specialised valve or series of valves installed at the wellhead of an oil or gas well to seal, control, and...
- Bow-Tie Analysis
- A risk-visualisation method combining a fault tree (left of the bow-tie, hazard to critical event) with an event tree (right, critical event...
- Latent Condition
- In Reason's model, a pre-existing organisational weakness (poor design, defective procedure, unrealistic scheduling, inadequate training) that lies dormant until aligned with other...
- Safety Management System (SMS)
- A formal, documented framework of policies, procedures, responsibilities, and performance monitoring through which an organisation identifies hazards, assesses risks, and manages safety...