How High Can Cohens D Be

Cohen-d’s go from 0 to infinity (in absolute value). Understanding it gets more complicated when you notice that two distributions can be very different even if they have the same mean.

Can Cohens d be 1?

Using this formula, here is how we interpret Cohen’s d: A d of 0.5 indicates that the two group means differ by 0.5 standard deviations. A d of 1 indicates that the group means differ by 1 standard deviation.

Can you have effect sizes greater than 1?

The value of the effect size of Pearson r correlation varies between -1 (a perfect negative correlation) to +1 (a perfect positive correlation). According to Cohen (1988, 1992), the effect size is low if the value of r varies around 0.1, medium if r varies around 0.3, and large if r varies more than 0.5.