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Bowtie Diagrams and the Human Factor

“For a long time, people were saying that most accidents were due to human error and this is true in a sense but it’s not very helpful. It’s a bit like saying that falls are due to gravity.” – Dr Trevor Kletz

There a several ways in which Bowtie Diagrams can be used to give us a greater understanding of how human error contributes to accidents. The first is by using threats. It can be tempting, and is quite common, to record ‘Human Error’ as a threat and then have barriers such as ‘competency’ or ‘training’, This is unlikely to tell the full story and isn’t very helpful. It can also give a false sense of security that there are multiple barriers in place when in fact, a failure of any one of these may result in an incident.

If human error is a legitimate cause of an accident scenario, then the specific error made should be described in the threat so that the reader understands. Then the barriers  can be more focused and refer to specific documents or procedures.

The second, and arguably more powerful mechanism for capturing human error, is the use of escalation factors. As described previously, escalation factors are threats to the performance of a barrier.

Consider a scenario where a barrier is that a procedure is followed correctly. An escalation factor may be that this procedure is not executed correctly. We know that there will be escalation barriers in place to prevent the barrier being degraded. The escalation barrier here is ‘Operator has necessary competence and time to follow procedure. The procedure is accurate and easy to follow’.  This clearly describes a number of measures that should take place: effort should be devoted to ensuring operators have demonstrable knowledge and competence to carry out the procedure; we carry out effective planning so they have the right environment to do so; the procedure is subject to regular review to ensure it remains accurate, and finally that it has been subject to a ‘Task Analysis’, helping to reduce the causes of failure.

We’re sure you’ll agree that this is far more helpful than simply writing ‘Human Error’.