According to the Swiss cheese model, which condition increases the likelihood of an error?

• A. Alignment of holes across layers.
• B. No weaknesses anywhere.
• C. All defenses are perfect.
• D. A single effective barrier.

Answer: (A)

In the Swiss cheese model, each layer of defense has its own weaknesses, or holes, that can let a hazard through. The risk of an error rises when those holes line up across multiple layers, creating a clear route for the hazard to bypass every barrier. So alignment of holes across layers is the situation that makes an error more likely. If there were no weaknesses anywhere, or if defenses were perfect, or if only a single barrier existed, there wouldn’t be a sequence of holes to align, and the likelihood of an error would be much lower.