One of the causes of the underuse of the Solomon four-group design may be that the complete details for the statistical analysis have not previously been presented. The primary issue previously unaddressed was how to combine an analysis of the effect of the treatment in the posttest-only groups with the same effect in the pre- and posttest groups (after an earlier phase of the analysis has shown no evidence of pretest sensitization.) A meta-analytic solution for this problem is proposed, and the entire analysis is presented, complete with flowchart and example. It is shown that the analysis has adequate statistical power even if the total N is not increased from that of a posttest-only design, removing the last of the serious objections to the Solomon design.

Almost 40 years ago, Solomon introduced a new form of experimental design typically referred to today as the Solomon four-group design (Solomon, 1949). Campbell and Stanley (1963) discussed this design as one of three one-treatment condition experimental designs, the other two being the pre- and posttest control group design and the posttest-only control group design (see Table 1). Each of these designs is adequate to assess the effect of the treatment and is immune from most threats to internal validity. The Solomon four-group design, however, adds the advantage of being the only one of the three able to assess the presence of pretest sensitization. Pretest sensitization means that “exposure to the pretest increases … the Ss’ sensitivity to the experimental treatment, thus preventing generalization of results from the pretested sample to an unpretested population”  ^{[  1  ]}  (Huck & Sandler, 1973, p. 54). Thus, the Solomon four-group design adds a higher degree of external validity in addition to its internal validity, and hence, according to Helmstadter (1970), is therefore “the most desirable of all the … basic experimental designs” (p. 110).

Despite its strength, however, the Solomon four-group design is underused. Four reasons may account for this underuse. First, the Solomon four-group design requires twice the number of groups used by...