Standardised mean difference
Short definition
The standardised mean difference is the difference between the means of two groups divided by the standard deviation, which allows studies that measured the same outcome on different scales to be compared and combined.
In plain language
If one study measures anxiety with a 0 to 80 questionnaire and another with a 0 to 10 scale, differences in points cannot be compared. Dividing by the variability of the data expresses the difference in standard deviations, a common unit. This is why it is the usual measure in meta-analyses of trials that used different instruments; Hedges' g is a version corrected for small samples.