Points to consider on the statistical analysis of rodent cancer bioassay data when incorporating historical control data

Researchers routinely use historical control data (HCD) when analyzing rodent carcinogenicity data obtained in a particular study. Although the concurrent control group is considered to be the most relevant group to compare with the dose groups, the HCD provides a broader perspective to assist in understanding the significance of the current study. The HCD is used to provide information about the incidences of spontaneous tumors and malignant systemic disorders such as lymphoma and leukemia. This article presents some possible ways of incorporating the HCD when analyzing data from a rodent cancer bioassay. Specifically, exploratory (informal) and formal statistical procedures for analyzing such data are reviewed. The boxplot is presented as an exploratory tool that describes the current data in the context of the distribution of the HCD. It will also identify potential outliers that would not be otherwise be flagged using standard methods such as the mean, standard deviation, and range. The various options for the statistical analysis of HCD presented here do not necessarily represent standard practice