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 A New Accident Model for Engineering Safer Systems

 by Nancy Leveson. Safety Science, Vol. 42, No. 4, April 2004. 


A new model of accidents is proposed based on systems theory. Systems are viewed as interrelated components that are kept in a state of dynamic equilibrium by feedback loops of information and control. Accidents result from inadequate control or enforcement of safety-related constraints on the system. Instead of defining safety management in terms of preventing component failure events, it is defined as a continuous control task to impose the constraints necessary to limit system behavior to safe changes and adaptations. Accidents can be understood, using this model, in terms of why the controls that were in place did not prevent or detect maladaptive changes, that is, by identifying the safety constraints that were violated and determining why the controls were inadequate in enforcing them. This model provides a theoretical foundation for the introduction of unique new types of accident analysis, hazard analysis, design for safety, risk assessment techniques, and approaches to designing performance monitoring and safety metrics.

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