Efficiency of age-adjusted tests in animal carcinogenicity experiments.

Carcinogenicity experiments may be analysed by a simple comparison of the control and exposed groups with respect to the proportions of observed tumors amongst dead animals, when longevity is identical in control and exposed groups. This simple-proportions test is invalid when longevity varies between groups and age-adjusted methods such as the Hoel-Walburg or logrank test are needed. This paper stresses that there is an advantage to using age-adjusted tests, even when the simple-proportions test is valid. The argument is based on calculations of the asymptotic relative efficiency of the simple-proportions test with respect to both the Hoel-Walburg and the logrank tests.