Which analogy did James Reason use to describe how accidents occur in a complete system?

• A. Holes in Swiss cheese
• B. Gaps in a fence
• C. Cracks in a dam
• D. Bubbles in glass

Answer: (A)

This focuses on James Reason's defense-in-depth concept, illustrated by the Swiss cheese model. In a complex system, safety relies on multiple barriers or defenses. Each barrier has its own weaknesses or holes, representing latent conditions or active failures. An accident occurs when holes in several barriers line up, creating a path through all defenses. The Swiss cheese image makes this visible: no single hole causes the disaster, but a matching alignment across several layers does. That’s why the analogy of holes in Swiss cheese best captures how accidents emerge from the interplay of multiple imperfect safeguards. Other analogies, like gaps in a fence, cracks in a dam, or bubbles in glass, imply a single flaw rather than the coordinated weaknesses across multiple layers that Reason emphasizes.