Fitting a model to categorical response data with application to species extrapolation of toxicity

The evaluation of toxicity data for noncarcinogens is complicated by the multiplicity of possible end points, and variations in both severity of effect and response rate. Often the response rates are not reported, so that "dose-response" analysis involves the relation between dose and severity of effect. Severity is usually reported as a description of the nature of the effects; measured values are rare. One approach is to then assign severity descriptions to ordered categories and to model the dose-category relationship. The application presented here is to interspecies scaling, i.e., the estimation of parameters in the model used to convert animal doses into equally toxic human doses.