Outcomes measured on a continuous scale - such as blood pressure, weight, age, or blood sugar - in treatment comparison groups are often summarised by calculating averages or means.
Treatment effects are then the difference in the averages of two treatment comparison groups.
In meta-analysis, the mean difference or weighted mean difference can also refer to the average effect across studies. The weight given to the average difference from each study (i.e. how much influence each study has on the overall results of the meta-analysis) is determined by the precision of the effect estimate.
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