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Accidents usually have multiple causes, whereby had any single one of those causes not happened, the accident would not have occurred. The British accident researcher James Reason describes this through the metaphor of slices of Swiss cheese: unless the holes all line up perfectly, there will be no accident. This metaphor provides two lessons: First, do not try to find “the” cause of an accident; Second, we can decrease accidents and make systems more resilient by designing them to have extra precautions against error (more slices of cheese), less opportunities for slips, mistakes, or equipment failure (less holes), and very different mechanisms in the different subparts of the system (trying to ensure that the holes do not line up).

The Swiss cheese metaphor suggests several ways to reduce accidents:

• Add more slices of cheese.

• Reduce the number of holes (or make the existing holes smaller).

• Alert the human operators when several holes have lined up.

Each of these has operational implications. More slices of cheese means mores lines of defense, such as the requirement in aviation and other industries for checklists, where one person reads the items, another does the operation, and the first person checks the operation to confirm it was done appropriately. Reducing the number of critical safety points where error can occur is like reducing the number or size of the holes in the Swiss cheese. Properly designed equipment will reduce the opportunity for slips and mistakes, which is like reducing the number of holes  and making the ones that remain smaller.

Excerpted from ‘The Design of Everyday Things’ by Don Norman