Effect size

Effect size is a statistical measure that quantifies the strength of the relationship between two variables on a numeric scale. This metric is used in statistical testing of the null hypothesis; usually, that the effect is zero. While sometime expressed in physical units (e.g. difference in blood pressure), effect sizes are more commonly ‘unit-free’ – such as Pearson’s correlation co-efficient (r), standardised mean difference (e.g. Cohen’s d) or regression coeffecients. For binary outcomes, relative risk (and relative risk reduction) is frequently used for effect size in clinical trials, and odds ratios are useful for combining the results of many studies in meta-analyses. In hypothesis testing, effect size, power, sample size and critical significance level are related to each other.