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Effect Size

  • By: Vicki Peyton
  • In: Encyclopedia of School Psychology
  • Edited by: Steven W. Lee
  • Subject:School/Educational Psychology (general), School Psychology, Educational Psychology

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]) ...

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