Regression models for the analysis of pretest/posttest data

The standard repeated measures ANOVA and ANCOVA models for data from pretest/posttest experiments may not be completely adequate either when a null pretest measurement implies that the posttest measurement is also null or when the data are heteroscedastic. We illustrate such a situation with an example in the field of dentistry involving the evaluation of children of both sexes with respect to a dental plaque index observed before and after toothbrushing with two types of toothbrushes. We propose a third alternative based on regression models having the post-toothbrushing index as response and the pre-toothbrushing index as explanatory variable, which may incorporate both the above requirements as well as the repeated measures nature of the data. Using the toothbrush data, we compare the results of the three analyses indicating how they can be implemented computationally.