Region 8

HH: Uncertainty Analysis

In this step, the risk assessor identifies and characterizes the main sources of uncertainty in the risk estimates presented in the risk characterization step. It is important to recognize that this assessment of uncertainty is not the same as the assessment of variability that is included in the Exposure Assessment section. Uncertainty is a consequence of imperfect knowledge or data and can (in theory) always be reduced by getting better data. In contrast, variability is an inherent property of an exposed population and cannot be changed by getting better data.

In most cases, the assessment of uncertainty is presented in a qualitative or semi-quantitative fashion, including a discussion of the likely direction and magnitude of the error associated with each important source of uncertainty. However, uncertainties may also be evaluated in a quantitative fashion using Probabilistic Risk Assessment (PRA) techniques. A fair and balanced characterization of uncertainty in risk estimates is especially important, because most risk estimates are not highly precise and many people are tempted to over-interpret the resulting values. A well-performed uncertainty analysis helps decision-makers and the public place the risk estimates in the proper perspective and facilitates informed decision-making.

Resources

RAGS I Part A (EPA/540/1-89/002, December 1989)

RAGS III Part A: Process for Conducting Probabilistic Risk Assessment (OSWER 9285.7-45, December 2001)

Guiding Principles for Monte Carlo Analysis (EPA/630/R-97/001, March 1997)

Policy for Use of Probabilistic Analysis in Risk Assessment at the U.S. Environmental Protection Agency (May 1997)

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