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Science Advisor Programs

Probabilistic Risk Assessment White Paper and Supporting Documents

Document History

Probabilistic Risk Assessment (PRA) began playing an increasingly important role in Agency risk assessments following the 1997 release of EPA's Policy for Use of Probabilistic Analysis in Risk Assessment at the U.S. Environmental Protection Agency and publication of the Guiding Principles for Monte-Carlo Analysis. Both the white paper and the companion FAQ document addresses recommendations on risk assessment processes described in the U.S. National Research Council’s (NRC) report Science and Decisions: Advancing Risk Assessment. The draft white paper and FAQ documents were released for public comment in September 2009 and underwent external peer review in May 2010. A 60-day public comments period closed October 16, 2009. Public comments are available online, under DOCKET ID NO. EPA-HQ-ORD-2009-0645. 


PRA is a group of techniques that incorporate variability and uncertainty into the risk assessment process. It provides estimates of the range and likelihood of a hazard, exposure or risk, rather than a single point estimate. It can provide a more complete characterization of risks, including uncertainties and variability, to protect more sensitive or vulnerable populations and lifestages. The information obtained from a PRA can be used by decision makers to weigh the risks of decision alternatives, or to invest in research with the greatest impact on risk estimate uncertainty.


These documents describe how PRA can be applied to enhance the scientific foundation of decision making across the Agency. They were created in response to recommendations from numerous advisory bodies, including the Science Advisory Board (SAB) and NRC, that EPA incorporate probabilistic analyses into the Agency’s decision-making process. This white paper explains how EPA can use probabilistic methods to address data, model and scenario uncertainty and variability by capitalizing on the wide array of tools and methods that comprise PRA. 
Both documents address issues such as variability and uncertainty, their relevance to decision making, and the PRA goal of providing quantitative characterization of the uncertainty and variability in estimates of hazard, exposure or risk. The difference between the white paper and the FAQs document is the level of detail provided about PRA concepts and practices and the intended audience (e.g., risk assessors vs. decision makers).