Mixed scale models for survival/sacrifice experiments

Information derived from interim sacrifices or on cause-of-death is routinely used in the statistical analyses of carcinogenicity experiments involving occult tumours. In this article, the authors describe a simple semi-parametric model which does not require this information. Natural deaths during the experiment and the usual terminal sacrifice provide sufficient information to ensure that the tumour incidence rates, which are of primary interest in occult tumour studies, can be estimated nonparametrically. The advantages of this semi-parametric approach to the analysis of survival/sacrifice experiments are illustrated using data from a study on benzyl acetate conducted under the U. S. National Toxicology Program. The results derived compare favourably with those obtained using a previously published approach to the analysis of tumorigenicity data.