Conducting power analyses for ANOVA and ancova in between-subjects designs

Researchers are frequently asked to justify the sample size used in their quantitative inquiries. Such a justification can be provided through a power analysis. Conducting power analyses, however, can raise some difficult issues regarding the specification of the size of the effect, testing for interaction effects, the role of covariates, and the use of an estimated effect size in the power analysis. The authors present methods for conducting power analyses along with a discussion of these issues, and they make available SAS programs that can be used to implement the power analyses that are discussed.