In statistics, an effect size is a measure of the magnitude of a treatment effect. It is an indicator of how important an obtained effect is. Unlike statistical significance tests, effect size does not depend on the sample size of an underlying study. It is helpful to report the effect size, not just the statistical significance, when assessing the effectiveness of a specific intervention in medical studies as well as studies in other sciences. It has been also widely used in meta-analysis, which combines and compares estimates from different but relevant studies.

In medical studies, such as comparison of a new treatment with other traditional ones, the following question is often asked: How well does the new treatment work? In answering this question, the researchers are ...

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