Cohen’s
d
a measure of effect size based on the standardized difference between two means: It indicates the number of standard deviation units by which the means of two data sets differ. For example, a mentoring intervention associated with a Cohen’s d of +0.25 indicates an increase of 0.25 standard deviation units for the average child who received mentoring relative to the average child who did not receive mentoring. The metric is used to represent effect sizes in meta-analysis as well as in the determination of power, with values of 0.20, 0.50, and 0.80 representing small, medium, and large effect sizes, respectively. See also Glass’s d. [Jacob Cohen (1923–1998), U.S. psychologist and statistician]