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 ver sus those for control group designs can account for the finding that SGPP d esigns yield larger mean effect sizes (e.g., M. S. Lipsey & D. B. Wilson, 1 993). It was found that the assumptions of no control group effect and the equivalence of pretraining and posttraining dependent variable standard dev iations required for these equations to produce equivalent estimates of eff ect size were violated for some dependent variable types. Results indicate that control group effects and inflation in the standard deviation of the p osttraining dependent variable measure account for most of the observed dif ference in effect size. The most severe violations occurred when the depend ent variable was a knowledge assessment. Methods for including data from SG PP designs in meta-analyses that minimize potential biases are discussed.