A hierarchical procedure for calculation of risk importance measures

Starting with a general importance definition based on conditional probabilities, a hierarchical process for calculating risk importance measures from a PSA's numerical results is developed. By the appropriate choice of events in the general definition, measures such as the risk achievement worth and the risk reduction worth can be calculated without requantifying the PSA's models. Required approximations are clearly defined and the subsequent constraints on the applicability of the process discussed. (orig.)