A Note on the Solomon 4-Group Design

Abstract

In analyzing the data associated with a Solomon Four-Group Design, the posttest scores are initially subjected to a 2x2 factorial ANOVA, with the two main effects being a.) pretest versus no pretest and b.) treatment versus no treatment. Campbell and Stanley (2) maintain that if this analysis yields non-significant F-ratios for both the main effect of pretesting and the pretesting-treatment interaction, it might be advantageous to reanalyze the data from the two pretested groups with an analysis of covariance (using pretest and posttest scores as the covariate and criterion variables, respectively). Assuming a high pretest-posttest correlation, the more powerful covariance might pick up a significant treatment effect which was not found in the initial analysis. Although Campbell and Stanley were correct in noting that this use of covariance must be preceeded by a non-significant pretesting-treatment interaction, the present authors argue that the covariance analysis is completely valid even if there is a true main effect for pretesting. A second point of this paper involves a recommendation for data analysis if the interaction from the initial two-way ANOVA does, in fact, turn out to be significant.