Effect size


Effect size (ES) is a name given to a family of indices that measure the magnitude of a treatment effect. Unlike significance tests, these indices are independent of sample size. ES measures are the common currency of meta-analysis studies that summarize the findings from a specific area of research. See, for example, the influential metaanalysis of psychological, educational, and behavioral treatments by Lipsey and Wilson (1993). There is a wide array of formulas used to measure ES. For the occasional reader of meta-analysis studies, like myself, this diversity can be confusing. One of my objectives in putting together this set of lecture notes was to organize and summarize the various measures of ES. In general, ES can be measured in two ways: a) as the standardized difference between two means, or b) as the correlation between the independent variable classification and the individual scores on the dependent variable. This correlation is called the "effect size correlation" (Rosnow & Rosenthal, 1996). These notes begin with the presentation of the basic ES measures for studies with two independent groups. The issues involved when assessing ES for two dependent groups are then described.

The psychotherapies include: behavioral treatments (primarily different forms of exposure therapies), eye movement desensitization and reprocessing (EMDR), relaxation therapy, hypnosis, and psychodynamic therapy. The control conditions include: pill placebo (used in the drug treatment studies), wait list controls, supportive psychotherapy, and no saccades (a control for eye movements in EMDR studies).






Lee A. Becker

Original Work Citation

Becker, L. (2000). Effect size. Author



“Effect size,” Francine Shapiro Library, accessed October 22, 2020, https://emdria.omeka.net/items/show/19134.

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