Cohen's d is a measure of effect size for two means. Which formula correctly defines d?

Cohen's d is a measure of effect size for two means. Which formula correctly defines d?

• A. d = (x1 − x2)/pooled SD
• B. d = (x1 − x2)/sample SD
• C. d = (x1 + x2)/pooled SD
• D. d = (x1 − x2)/standard error

Answer: (A)

Cohen’s d expresses how large the difference between two group means is, in units of variability. It standardizes the raw difference by dividing by a measure of spread, so the result is dimensionless and comparable across studies. The standard way uses the pooled standard deviation, which combines the within-group variability from both groups to estimate the common dispersion when the groups have similar variances. This makes d reflect the separation of the means relative to typical within-group spread. The standard error would mix in sampling variability rather than the actual dispersion, and using a sum of means isn’t a measure of difference at all. So the correct form is the difference of means divided by the pooled standard deviation.

Cohen’s d expresses how large the difference between two group means is, in units of variability. It standardizes the raw difference by dividing by a measure of spread, so the result is dimensionless and comparable across studies. The standard way uses the pooled standard deviation, which combines the within-group variability from both groups to estimate the common dispersion when the groups have similar variances. This makes d reflect the separation of the means relative to typical within-group spread. The standard error would mix in sampling variability rather than the actual dispersion, and using a sum of means isn’t a measure of difference at all. So the correct form is the difference of means divided by the pooled standard deviation.