Bayesian analysis improves functional safety

摘要

Functional safety engineers follow the ISA/IEC 61511 standard and perform calculations based on random hardware failures. These result in very low failure probabilities, which are then combined with similarly low failure probabilities for other safety layers, to show that the overall probability of an accident is extremely low (e.g., 1E-5/yr). Unfortunately, such numbers are based on frequentist assumptions and cannot be proven. However, looking at actual accidents caused by control and safety system failures shows that accidents are not caused by random hardware failures. Accidents are typically the result of steady and slow normalization of deviation (a.k.a. drift). Although it is up to management to control these factors, Bayes' theorem can be used to update our prior belief (the initial calculated failure probability) based on observing other evidence--such as the effectiveness of the facility's process safety management process. The results can be dramatic.