Effect Size

An effect size refers to the magnitude of the impact of treatment on an outcome measure. There are two broad families of effect size indexes: the standardized mean difference and measures of association (Kline, 2004). Both compare results across different studies or variables measured in different units and each is a first step toward evaluating the practical importance of a research finding.

For example, suppose that the same two treatments are compared in two different studies. The outcome variable in each study reflects the same construct, off-task behavior, but the standard deviation (the square root of the average squared distance of a set of scores from their mean or average [i.e., the sum of all the scores divided by the number of scores in the set]) ...