Statistical tests of significance in transgenic mutation assays: considerations on the experimental unit

When significant animal-to-animal variability is present in binary response data, the usual statistical tests applied to such data do not always operate correctly. In transgenic mouse mutation data, some evidence of significant animal-to-animal variability already exists, suggesting that conventional statistical methods may not be appropriate. Here, we describe an alternative statistical method that treats the animal as the experimental (or statistically independent) unit, and contrast results of its application with those from methods that take the transgene as the experimental unit. Using data from two publications that report experimental results for individual animals, the transgene-based and animal-based analyses can yield very different interpretations of the experimental data. The performance of animal-based statistical methods should be improved by conducting future experiments with enough animals to adequately address animal-to-animal variability.