Probabilistic risk assessment (PRA), in its simplest form, is a group of techniques that incorporate variability and uncertainty into risk assessments. Variability refers to the inherent natural variation, diversity and heterogeneity across time, space or individuals within a population or lifestage, while uncertainty refers to imperfect knowledge or a lack of precise knowledge of the physical world, either for specific values of interest or in the description of the system (USEPA 2011c). Variability and uncertainty have the potential to result in overestimates or underestimates of the predicted risk.
PRA provides estimates of the range and likelihood of a hazard, exposure or risk, rather than a single point estimate. Stakeholders inside and outside of the Agency have recommended a more complete characterization of risks, including uncertainties and variability, in protecting more sensitive or vulnerable populations and lifestages. PRA can be used to support decision‐making risk management by assessment of impacts of uncertainties on each of the potential decision alternatives. The aim of the document is to summarize the Risk Assessment Forum White Paper: Probabilistic Risk Assessment Methods and Case Studies (USEPA 2014a) for managers and agency scientists to gain a better understanding of the principles of PRA without the more detailed discussion presented in the White Paper.
Numerous advisory bodies, such as the Science Advisory Board (SAB) and the National Research Council (NRC) of the National Academy of Sciences (NAS), also recommended that EPA incorporate probabilistic analyses into the Agency’s decision‐making process. EPA’s Risk Assessment Forum (RAF) formed a Technical Panel, consisting of representatives from the Agency’s program and regional offices, to develop this Frequently Asked Questions (FAQ) document and its companion publication titled Risk Assessment Forum White Paper: Probabilistic Risk Assessment Methods and Case Studies (USEPA 2014a). The RAF is also recommending the development of Agency resources, such as a clearinghouse of PRA case studies, best practices, resources and seminars, to raise general knowledge about how these probabilistic tools can be used.
The purpose of this FAQ document is to present general concepts and principles of PRA, describe how PRA can improve the bases of Agency decisions, and provide illustrations of how PRA has been used in risk estimation and in describing the uncertainty in decision making. This document is a companion to the more detailed white paper, which also includes a compendium of 16 relevant case studies ranging in their level of complexity. Both documents describe the potential and actual uses of probabilistic tools in the risk decision process and facilitate their implementation in human, ecological and environmental risk analysis and related decision‐making at EPA. The documents do not prescribe a specific approach to risk assessment; rather, they describe the various stages and aspects of an assessment or decision process in which probabilistic tools may add value. These are not regulatory guidance documents, but they are designed to be helpful handbooks and/or references for risk assessors, risk managers and decision makers interested in improving the risk assessment and decision making process.
Overall, this FAQ document answers more than 20 questions commonly asked about PRA, beginning with “Why should I care about PRA, and why is it important to risk managers?” and ending with “How can I get more information on PRA?” These questions cover a wide range of PRA issues, and risk managers, decision makers and other interested readers will find a plethora of PRA information in this document. Further detailed information, including recommendations for future activities, are provided in the companion white paper (USEPA 2014a).