This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  The mean difference (or difference in means)

The mean difference (more correctly, ‘difference in means’) is a standard statistic that measures the absolute difference between the mean value in two groups in a clinical trial. It estimates the amount by which the experimental intervention changes the outcome on average compared with the control. It can be used as a summary statistic in meta-analysis when outcome measurements in all studies are made on the same scale.


Aside: Analyses based on this effect measure have historically been termed weighted mean difference (WMD) analyses in the Cochrane Database of Systematic Reviews (CDSR). This name is potentially confusing: although the meta-analysis computes a weighted average of these differences in means, no weighting is involved in calculation of a statistical summary of a single study. Furthermore, all meta-analyses involve a weighted combination of estimates, yet we do not use the word ‘weighted’ when referring to other methods.