, Volume 9, Issue 3, pp 241-249

A Visitor's Guide to Effect Sizes – Statistical Significance Versus Practical (Clinical) Importance of Research Findings

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Effect Sizes (ES) are an increasingly important index used toquantify the degree of practical significanceof study results. This paper gives anintroduction to the computation andinterpretation of effect sizes from theperspective of the consumer of the researchliterature. The key points made are:1. ES is a useful indicator of the practical(clinical) importance of research resultsthat can be operationally defined frombeing ``negligible'' to ``moderate'', to``important''.2. The ES has two advantages overstatistical significance testing: (a) itis independent of the size of the sample;(b) it is a scale-free index. Therefore,ES can be uniformly interpreted indifferent studies regardless of the samplesize and the original scales of thevariables.3. Calculations of the ES are illustrated byusing examples of comparisons between twomeans, correlation coefficients,chi-square tests and two proportions,along with appropriate formulas.4. Operational definitions for the ESs aregiven, along with numerical examples forthe purpose of illustration.