Risk implies uncertainty. ERA was designed expressly to include uncertainty as an integral component of the assessment process. Sources of uncertainty include natural variability in ecological and environmental phenomena, as well as bias and imprecision associated with the exposure-response functions. This latter source of uncertainty can be exacerbated if extrapolations were involved in the derivation ofthe functions (e.g., laboratory to field, across species). Incomplete and imperfect understanding of baseline ecological phenomena also adds uncertainty to ERA.

Uncertainties inherent to the risk assessment process can be quantitatively described using, for example, statistical distributions, fuzzy numbers, or intervals. Corresponding methods are available for propagating these kinds of uncertainties through the process of risk estimation, including Monte Carlo simulation, fuzzy arithmetic, and interval analysis. Computationally intensive methods (e.g., the bootstrap) that work directly from the data to characterize and propagate uncertainties can also be applied in ERA. Implementation of these methods for incorporating uncertainty can lead to risk estimates that are consistent with a probabilistic definition of risk.

Methods of numerical sensitivity and uncertainty analysis can be used to examine uncertainty and identify the key sources of bias and imprecision in quantitative estimates of risk. Once identified, limited resources (e.g., time, funding) can be efficiently allocated to obtain new information and data for those major sources of uncertainty and reduce uncertainty. These analyses can be repeated until uncertainties associated with the risk estimates are of an acceptable degree or until uncertainties cannot be further reduced. Importantly, application of these methods for analyzing uncertainty will identify whether unacceptably high estimates of risk derive from the inherent severity of the stressor or from high uncertainty.

See also: Bioaccumulation; Bioavailability; Biodegradability; Biodegradation; Photolysis.

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