Sunday, April 6, 2008

Error Chains and Swiss Cheese

Most mariners are familiar with the error chain. One study found that the number of causes (links) of an accident ranged from 7 to 58 with an average of 23. The thinking is, identify one link in the error chain, remove it, and the error chain is broken, thus, no incident.

How does one identify these links? After the incident, one can trace the chain, in retrospect, finding and removing one link seems obvious. In practice however, the error chain concept provides little guidance in finding and breaking the chain.

A more helpful model is the Swiss Cheese Model first proposed by British psychologist James T. Reason (nice name!).

From Wikipedia
"most accidents can be traced to one or more of four levels of failure: Organizational influences, unsafe supervision, preconditions for unsafe acts, and the unsafe acts themselves. In the Swiss Cheese model, an organization's defenses against failure are modeled as a series of barriers, represented as slices of Swiss cheese. The holes in the cheese slices represent individual weaknesses in individual parts of the system, and are continually varying in size and position in all slices. The system as a whole produces failures when all of the holes in each of the slices momentarily align, permitting (in Reason's words) "a trajectory of accident opportunity", so that a hazard passes through all of the holes in all of the defenses, leading to a failure"

This site has a nice graphic of the S.C. model.

The Swiss Cheese Model provide a positive method of reducing risk, rather then seeking to break some invisible chain, one simply adds layers, or increases the effectiveness of the existing layers (making the holes smaller). As an example, one could add a layer of crew training, or seek ways to improve the effectiveness of existing training, or use additional care during passage planning.

On a well run ship you can observe the Swiss Cheese Model in action Each near miss, representing a hole in one layer, is evaluated and if needed procedures are modified. Near misses, lessons learned, Bridge Resource Management, careful passage planning can all be seen as adding layers of cheese. Of course it may not be called that. Another name for the application of the Swiss Cheese model is - good seamanship.

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