ABSTRACT
Virtually all accident investigations and risk assessments are conducted in a state of relative ignorance of the full behaviour of the system. This condition contrasts with the fact that all established approaches to risk assessment require that it is possible to describe the system and the scenarios in detail; that is, that the system is tractable. Systems are usually defined with reference to their structure, that is, in terms of their parts and how they are connected or put together. Common definitions emphasise both that the system is a whole, and that it is composed of independent parts or objects that are interrelated in one way or another. One important development in the history of industrial safety was the transition that happened in response to the accident at the Three Mile Island nuclear power plant in 1979. A similar transition took place when the focus changed from the human factor to the organisation and/or safety culture.
