Cohen’s *d* is a common measure of effect size (ES).

The basic formula for *d* is:

This corresponds to the differences between populations means divided by the population standard deviation.

Cohen offered the following rough guidelines to describe effect sizes: a small ES = .2, a medium ES = .5, and a large ES = .8 and above.

It is important to recognize that the size of an effect does not necessarily tell us about the importance of the effect. For example, the effect of aspirin on reducing heart attacks is an important effect but the effect size is less than 0.10.

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