Summary: In certain cancer chemoprevention experiments both the number of observed tumors per animal and their times to detection are used in subsequent statistical analyses. The mathematical models used to represent these experiments usually include the Poisson distribution to characterize the tumor multiplicity data. Very often however, there is excess variance due to interanimal heterogeneity of tumor response. Thus, the number of induced tumors is better characterized by the negative binomial distribution.
We modify an existing statistical technique, which explicitly acknowledges the confounding inherent in these systems, in order to provide a more efficient procedure for utilizing the information in a sample and to more accurately assess treatment effects.