What Are Effect Sizes and Why Do We Need Them?


Correspondence concerning this article should be addressed to Larry V. Hedges, Department of Statistics, Northwestern University, 2040 N. Sheridan Road, Evanston, IL 60208; e-mail: l-hedges@northwestern.edu.


ABSTRACT—Effect sizes are quantitative indexes of the relations between variables found in research studies. They can provide a broadly understandable summary of research findings that can be used to compare different studies or summarize results across studies. Unlike statistical significance (p values), effect sizes represent strength of relationships without regard to sample size. Three families of effect sizes are widely used: the standardized mean difference family, the standardized regression coefficient family, and the odds ratio family.