SOLOMON FOUR GROUP DESIGN

This design is a combination of the pretest-posttest control group design and the posttest only control group design.  It is as good at controlling threats to internal and external validity as the posttest only control group design, but superior to that design with respect to statistical conclusion validity.  However, it gains that advantage at the cost of considerably greater expense and effort in the collection and analysis of the data.  

To test the hypothesis of no testing by treatment interaction, one could arrange all four groups’ posttest scores into a 2 x 2 factorial ANOVA design, (pretested or not) X (given the experimental treatment or not).  A significant interaction would indicate that the simple main effect of the treatment among those that were pretested was not the same as the simple main effect of the treatment among those that were not pretested.  The main effect of pretesting and the main effect of the treatment could also be assessed with such an ANOVA.  If there existed a significant testing by treatment interaction, one could test the simple main effect of the treatment for the pretested subjects and for the not pretested subjects.  One might also want to analyze the data from the pretest-posttest control group part of the design with those techniques which are appropriate for such a design (such as ANCOV on the posttest scores using pretest scores as a covariate).

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