A comparison of methods for increasing power in randomized between-subjects designs

Power in a randomized between-subjects design can be enhanced by increasing sample size or increasing alpha level. This article compares 2 other methods of increasing power by reducing within-group error: (a) by adding a pretest to the design and. using analysis of covariance (AN COVA) and (b) by increasing the length of the posttest and using analysis of variance (ANOVA). Results showed that the relative power of these approaches depends on the degree to which the posttest is lengthened, on the reliability of the posttest, and on the pretest-posttest correlation. When reliability or the pretest-posttest correlation is low, doubling the length of the posttest ANOVA more powerful than ANCOVA conducted on the original measures.