Purpose and Mindset

The purpose of an incident investigation is to understand how the organisation's defences failed, so they can be improved. It is not to establish fault, and an investigation aimed at fault will find it and stop there.

The assumption that ruins investigations

"Human error" as a conclusion. People do not come to work intending to be injured, and in almost every case the action that looks obviously wrong afterwards looked entirely reasonable at the time, given the information, the time and the pressures the person had. If your investigation concludes that someone made a mistake, you have found the start of the investigation, not the end: why did that action make sense to them then?

Hindsight bias

Knowing the outcome makes the warning signs obvious and the correct choice clear. Investigators must deliberately reconstruct what the people involved actually knew, when they knew it, and what else was competing for their attention. The test: could a competent, well-intentioned person in the same situation, with the same information, have done the same thing? If yes, the cause is situational, not personal.

Scope

Set the scope deliberately: the event, the period before it that is relevant, the systems and decisions in scope, and explicitly whether organisational and management decisions are included. An investigation scoped to the workface can only find workface causes.