A standardized measure of effect size that expresses the difference between two group means in terms of their pooled standard deviation. Values of 0.2, 0.5, and 0.8 are considered small, medium, and large effects respectively.
Named after Jacob Cohen who systematized effect size interpretation in his influential 1988 book 'Statistical Power Analysis for the Behavioral Sciences.' The 'd' simply denotes this as Cohen's measure of standardized mean difference, distinguishing it from other effect size measures.
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