This chart describes a model of resilience assurance, based on the Swiss Cheese metaphor ... proposed by James Reason.

Resilience is about protecting from hazards, namely, preventing them from resulting in incidences. A resilient system is analogous to a stack of slices of Swiss cheese, each slice representing a defense layer against potential hazards.

• Hazard A in the chart is blocked by the first defense layer.

Each defense layer may provide protection by blocking the hazard propagation, therefore, the more defenses we put up, the better.

• Hazard B in the chart passes through all the defense layers, resulting in an incidence.

However, the defenses cannot provide full protection. The risks of hazards developing to incidences are represented by holes in the slices of the Swiss cheese.

A hazard passing through a hole is a threat. For an incidence to occur, the holes need to align for all defenses, so that the threats can propagate through the holes. Therefore, in the process of  resilience assurance we design add-ons, enabling blocking the hazard propagation.

• Hazard C in the chart passes through the first defense layer before it is blocked by an add-on built in the second layer.

Add-ons are not always perfect, and they not always provide sufficient protection.

• Hazard D in the chart passes through holes in the first two defense layers, then it goes through an ill-designed add-on built in the third layer and finally is successfully blocked by an add-on in the last layer.

Updated on 31 Mar 2017.