On the relationship between safety and decision significance

Risk analysts are often concerned with identifying key safety drivers, i.e., the systems, structures and components (SSCs) that matter the most to safety. SSCs importance is assessed both in the design phase (i.e., before a system is built) and in the implementation phase (i.e., when the system has been built) using the same importance measures. However, in a design phase it would be necessary to appreciate whether the failure/success of a given SSC can cause the overall decision to change from accept to reject (decision significance). This work addresses the search for the conditions under which SSCs which are safety significant are also decision significant. To address this issue, the work proposes the notion of $\theta-$importance measure. We study in detail the relationships among risk importance measures to determine which properties guarantee that the ranking of SSCs does not change before and after the decision is made. An application to a Probabilistic Safety Assessment model developed at NASA illustrates the risk management implications of our work.