The difference between two means, divided by an estimate of the within-group standard deviation.
Note: When a continuous variable (such as pain) is measured in a variety of ways across studies (using different scales), it may prove impossible to compare or combine the study results in a meta-analysis. If the effects are expressed as a standardised value, the results can be combined, since they are no longer expressed as units of measurement. Standardised mean differences are sometimes referred to as a “d index.”*
*Concept introduced by J. Cohen (1988).
Alternative spelling: standardized mean difference (SMD)