Effect Size
Definition
Effect size is a quantitative measure of the magnitude of a difference or association observed in a study.
Correct Scientific Usage
Effect size describes how large an effect is, independent of whether the result is statistically significant. Researchers report effect sizes to convey practical significance. Confidence intervals around effect sizes provide crucial information about precision and uncertainty.
Common Misunderstandings
Most people focus exclusively on whether results are statistically significant while ignoring effect sizes entirely. People may also dismiss important findings because they’re not statistically significant, not realizing the effect size might be substantial but the study underpowered.
Why It Matters
Effect sizes are essential for evaluating whether research findings have practical importance beyond statistical detectability. They enable meaningful comparisons between different interventions and studies.
Understanding effect sizes prevents being misled by statistically significant results that don’t matter in practice, and helps recognize when non-significant results might still represent important effects.
References
- Using the Confidence Interval Confidently, Journal of Thoracic Disease
- Using Effect Size—or Why the P Value Is Not Enough, Journal of Graduate Medical Education
Related Terms
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