This article examines whether differences in the questions commonly used to calculate effect size for single group pretest-posttest (SGPP) designs versus those for control group designs can account for the findings 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 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 measure account for most of the observed differences 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 minimise potential biases are discussed. (A)
Samenvatting