in a meta-analysis, a measure of the average effect size across multiple studies. For example, an investigator analyzing several studies assessing a new treatment may determine a mean effect size by calculating the average standardized difference between treatment and control groups over all of the studies. Calculations of mean effect size often take variance and reliability into account by assigning different weights to the values derived from different studies. A study with a large sample, for instance, or one that uses more precise measurement techniques may be weighted to have a greater impact in determining the mean effect size.