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

Abstract

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. In an extensive review of psychological, educational, and behavioral treatment research, Lipsey and Wilson (1993) examined results from meta-analyses reporting effect size estimates for studies subgrouped by research design. Data from studies employing single group pretest-posttest (SGPP) designs were compared with data from studies using control or comparison group designs (i.e., pretestposttest with control, PPWC; and posttest only with control, POWC). Across 45 meta-analyses , mean effect sizes (dvalues) for SGPP designs were 0.29 SDs larger (0.76 versus 0.47) than effect sizes for studies using control or comparison group designs. Research by Guzzo, Jette, and Katzell (1985) has found similar differences. The common conclusion among these researchers is that data from SGPP designs clearly are capable of upwardly biasing the mean treatment effect estimates derived from meta-analysis (Lipsey & Wilson, 1993, p. 1194). Lipsey and Wilson's (1993) solution to the perceived upward bias in effect size for SGPP designs was not to