Effect Sizes Based on Means

When the outcome is reported on a meaningful scale and all studies in the analysis use the same scale, the meta-analysis can be performed directly on the raw difference in means (henceforth, we will use the more common term, raw mean difference). The primary advantage of the raw mean difference is that it is intuitively meaningful, either inherently (for example, blood pressure, which is measured on a known scale) or because of widespread use (for example, a national achievement test for students, where all relevant parties are familiar with the scale). Consider a study that reports means for two groups (Treated and Control) and suppose we wish to compare the means of these two groups. Let 1 and 2 be the true (population) means of the two groups. The population mean difference is defined as