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March 1998 SAP Meeting Summary

The purpose of this Guidance document is to establish standards for submissions of probabilistic human health exposure assessments to the Health Effects Division (HED) of the Agency's Office of Pesticide Programs (OPP). This document is designed to provide interim guidance on general concepts for the preparation and review of probabilistic assessments. It is not intended to provide step-by-step instructions on conducting probabilistic assessments. As HED gains greater experience in reviewing such assessments, more detailed guidance will be developed.

Agency policy is that risk assessment should be conducted in a tiered approach, proceeding from simple to more complex analyses as the risk management situation requires (Agency Policy Document, 5/15/97). More complex analyses require greater resources, and probabilistic assessments can represent high levels of complexity. In a deterministic assessment, exposure is expressed as a single value, which could represent an upper-bound scenario (for example, tolerance levels on foods for dietary exposure), or a statistical tendency (for example, average values from appropriate field trial data). If a deterministic analysis based on conservative assumptions leads to risk estimates that are below levels of concern, then there is no need to refine risk assessments with more complex techniques.

In contrast to deterministic techniques, probabilistic risk assessments more fully consider ranges of values regarding potential exposure, and then weight possible values by their probability of occurrence. Individual input values used to generate a point estimate are replaced by a distribution reflecting a range of potential values; a computer simulation then repeatedly selects individual values from each input distribution to generate a range and frequency of potential exposures. The output is a probability distribution of estimated exposures, from which exposures at any given percentile can be determined. Consideration of probabilistic analyses will be limited to exposure assessments. In accordance with Agency policy at the current time, such techniques will not be considered for dose-response evaluations of toxicological data.

Scientific Advisory Panel (SAP) March 1998 Meeting Summary

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