Impact of experimental design on effect size: Findings from the research literature on training

This article examines whether differences in the equations commonly used to calculate effect size for single group pretest-posttest (SGPP) designs versus those for control group designs can account for the finding that SGPP designs yield larger mean effect sizes (e.g., M. S. Lipsey & D. B. Wilson, 1993). It was found that the assumptions of no control group effect and the equivalence of pretraining and posttraining dependent variable standard deviations required for these equations to produce equivalent estimates of effect size were violated for some dependent variable types. Results indicate that control group effects and inflation in the standard deviation of the posttraining dependent variable measure account for most of the observed difference in effect size. The most severe violations occurred when the dependent variable was a knowledge assessment. Methods for including data from SGPP designs in meta-analyses that minimize potential biases are discussed.