On the treatment of dependence in making decisions about risk

Much attention has been paid to the treatment of dependence in performing probabilistic risk assessments (PRA). For instance, causal dependencies (e.g., common cause failures, cascade failures, and intersystem dependencies) have been taken into account in PRAs beginning with the Reactor Safety Study (USNRC, 1975). In addition, beginning in the early 1980s, attention began to be paid to the issue of probabilistic dependence between the failure rates (Apostolakis and Kaplan, 1981) or seismic fragilities (Kaplan, 1985) of similar components, and the impact of such dependence on risk estimates. By now, it has been clearly demonstrated that failure to take either casual or probabilistic dependence into account in PRAs can lead to misleading results, typically underestimates of the true risk. However, there has been little attention to date on the effects of dependence in the area of decision making. The objectives of this paper are to illustrate the potential importance of dependence in making decisions about risks, and to present some ideas on how to communicate the effects of dependence to decision makers in a clear and easily comprehensible manner