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Issues in Risk Assessment Field-scale experiments could be used to validate specific risk assessment models keyed to specific pesticide-application scenarios. Polychlorinated Biphenyls and 2,3,7,8-Tetrachlorodibenzo-p-Dioxin This case study was an excellent example of the use of models to predict the environmental fate of persistent chemical contaminants. It included evaluations of uncertainty and descriptions of validation studies. In formal risk assessments, the models described are used to estimate spatiotemporal profiles of environmental contamination in sediment, water, and fish that are then compared with regulatory standards. Dose-response relationships are not used. Species Introductions Models are not currently part of the regulatory framework for USDA's species introduction program. However, the author of the case study discussed research in which models of host-parasite dynamics are being used to examine the potential effectiveness of control agents proposed for introduction. This approach was viewed by the group as being analogous to defining dose-response relationships. Some attempts are being made to use models to extrapolate from test environments to other environments of interest, but models are not used for risk characterization. No methods exist for evaluating the impact of species introductions on a regional scale. Northern Spotted Owl The consensus of the group was that the case study was an example of hazard identification, rather than of complete risk assessment. There was no characterization of risk or uncertainty. Models might be useful for simulating the various factors that determine spotted owl population viability. A full-scale owl population model—incorporating resource availability, habitat suitability, and competitive interaction—would allow