It’s the Effect Size, Stupid: What effect size is and why it is important (2002)
What effect size is and why it is important
Paper presented at the Annual Conference of the British Educational Research Association, University of Exeter, England, 12-14 September 2002
Effect size is a simple way of quantifying the difference between two groups that has many advantages over the use of tests of statistical significance alone. For example, an effect size of 0.8 means that the score of the average person in the experimental group is 0.8 standard deviations above the average person in the control group, and hence exceeds the scores of 79% of the control group. Notice that an effect-size of 1.6 would raise the average person to be level with the top ranked individual in the control group, so effect sizes larger than this are illustrated in terms of the top person in a larger group.
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