Effect Size

The magnitude of improvement is reported as effect size. The use of the effect size statistic, also known as Cohen’s d, is important for benchmarking results since it provides a common metric independent of the questionnaire used (Cohen, 1988). ES is calculated as pre-post change divided by the standard deviation of the outcome measure at intake.

Most journals today require the use of effect size when reporting results. Many decades of research on psychotherapy outcomes yields an average effect size for psychotherapy of approximately 0.8. For this reason, we have classified effect sizes of .8 or greater as indicative of “highly effective” treatment.

An effect size of 1 means that the patient improved the equivalent of one standard deviation on the outcome measure. Effect sizes of 0.8 or higher are considered large, while effect sizes of 0.5 to 0.8 can be considered moderately large. Effect sizes of less than 0.3 are small and might well have occurred without any treatment at all.

The American Psychological Association has been officially encouraging authors to report effect sizes since 1994 - but with limited success."

-Robert Coe, Address to the British Educational Research Association.