No theory forbids me to say "Ah!" or "Ugh!", but it forbids me the bogus theorization of my "Ah!" and "Ugh!" - the value judgments. - Theodor Julius Geiger (1960)

Charles Perrow (1925-2019) was an American sociologist who wrote primarily about the impact of complex organizations on society.

Perrow changed the way we look at system accidents. He starts his book Normal Accidents (1984) with some definitions.
- An accident is a failure of a subsystem, or the system as a whole, that damages more than one unit, thereby disrupting the current or future output of the system.

- An incident is damage that is limited to parts or a unit, regardless of whether the failure causes the output to decrease and requires repairs or not.

The nature of systems causes the damage. The failure of parts and units plays a crucial role in the failure of subsystems and systems, but if the analysis is limited to that, we lose focus on the kinds of systems that business and government leaders decide should be built. Falling from a ladder is therefore just an incident for Perrow and not an accident. He is concerned about those systems that can harm a large number of people. The number of OHS incidents can be reduced through everyday precautions and training.

Most safety and accident work has to do with first-party victims and to some extent second-party victims. But Perrow is concerned about third- and fourth-party victims. In short, the first victims are the operators; secondary victims are non-operating personnel or system users such as passengers on a ship; third-party victims are innocent bystanders; fourth party victims are fetuses and future generations. This is about more and more people and the risks are increasingly unknown and less well managed. In that case, e.g. parts, or units, or subsystems have multiple functions. The addition of redundant components, seen as the main line of defense, turns out also the main cause of the failures. More components and more complexity in system design creates complex interactions, branch paths, feedback loops, jumps from one linear series to another.

Much more general interactions, the kind we try to construct intuitively because of their simplicity and comprehensibility, Perrow calls linear interactions. Linear interactions predominate in all systems, but even the most linear systems will have at least one source of complex interactions, the environment, because it affects many parts or units in the system. Only the environment can be a source of interference that is common to many components - a common mode accident.

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