The use of repeated measures analyses in developmental toxicology studies.

In many toxicology studies, it is common to take the same measurements on an individual animal at several time points (e.g., body weight across days or weeks, activity levels either within a single test session or across days). Such repeated measures for both maternal and offspring endpoints routinely are incorporated into developmental toxicology studies to allow characterization of the profile of treatment-related effects over time. To permit a valid statistical analysis, repeated measures studies are characterized by: random assignment of treatment to experimental units, measurement of the variable(s) of interest at the same time points for all animals, and avoidance of systematic overlaying of treatment and potential response gradients. Assumptions for and advantages of the use of a repeated measures analysis of variance versus single-factor analyses at each time point are illustrated in two data sets from a rat perinatal and postnatal study. The variance-covariance structure of repeated measures designs dictates that adjustments are necessary to provide protection against inflated Type 1 error rates. Communication between statisticians and toxicologists which allow the implementation of such analyses can improve the interpretation of data resulting from repeated measures study designs.