Ashby’s law of requisite variety

The defences-in-depth philosophy presented in Section 7.1 can also be interpreted in terms of Ashby s law of requisite variety. Here, the system s designer has the clear ambition of preventing major accidents through design of barriers. This will make it less demanding for the operations organisation to manage the major accident risks. 

There is another important aspect of Ashby s law of requisite variety. It has to do with the rate of change or variety in a system to be controlled. In this sense, Ashby s law of requisite variety reads For an analyst to gain control over a system, he must be able to generate countermeasures at least at a rate corresponding to the rate of variety that the observed system can exhibit. ... 

To sum up, Ashby s law of requisite variety implies that an analyst must show the following three abilities in order to exercise full control of a system ... 

The requirement for adequate coverage comes from Ashby s law of requisite variety. To exercise efficient control, we must receive data on the different technical, organisational and human factors that affect the risk of accidents and are controllable through management decisions. It also means that data collection must be flexible enough to handle unexpected threats and changes. 

Let us analyse these findings in relation to Ashby s law of requisite variety. This law conveys an important message concerning the necessary conditions for us to be able to exercise full control of a system. If the available information on accident risks has a limited coverage, it will limit the variety of the action alternatives taken to control accident risks. The variety of the actions may well fall below the critical level the accident risk will remain at the same level or even increase as a result. Many opportnnities for accident control are lost if the investigator fails to identify how prodnction disturbances affect the risk of accidents. 

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