The results of comparative clinical studies can be expressed using various intervention effect measures. Examples are absolute risk reduction (ARR), relative risk reduction (RRR), odds ratio (OR), number needed to treat (NNT), and effect size. The appropriateness of using these to express an intervention effect, and whether probabilities, means, or medians are used to calculate them, depend upon the type of outcome variable used to measure health outcomes. For example, ARR, RRR and NNT are used for dichotomous variables and effect sizes are normally used for continuous variables.
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