Uncertainty Analysis - trdoer40103

Uncertainty Analysis

Uncertainty is introduced into each step of the risk assessment process. The final

risk estimates represent the integration of selected pieces of information, each with

its own degree of uncertainty. To address this uncertainty, the risk assessment

makes conservative assumptions about potential exposures and toxicity. Therefore,

the predicted risk estimates may overestimate actual risks. It is important to

recognize that risk estimates are indicators of the potential for adverse effects, not

predictors of such effects.

In a risk assessment, there are two ways to describe uncertainty, quantitatively

and qualitatively. For most dredged material management activities, uncertainty

characterization will typically involve a qualitative discussion of the rationale for

using particular scenarios, exposure factors, and data and the level of confidence in

those selected parameters. The larger, more complex assessments will require a

more quantitative process.

It is possible to express the uncertainty by running the exposure scenarios under

various alternative assumptions. These may range from using different statistics for

EPCs, varying the frequency of exposure, or changing assumptions regarding the

characteristics of the exposure for each scenario. This should be done within the

framework of the agreed upon scenarios, and not result in new or separate scenarios

involving new receptors, contaminants, or previously unconsidered databases.

The risk assessment should include a qualitative uncertainty characterization that

identifies site-related variables and assumptions that contribute to the overall

uncertainty in the risk estimates. The uncertainty analysis should:

a. Identify likely sources of uncertainty in the risk assessment.

b. Identify clearly all significant assumptions at each stage of the assessment.

c. Identify the range and, where possible, the distribution of values which a

parameter may take.

d. Test the sensitivity of the risk assessment by using the bounding values for

these assumptions for the most uncertain assumptions.

Chapter 5 Uncertainty Analysis