[Four numbers and a bit more basic knowledge of mathematics].
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In addition to relative risk, relative risk reduction and absolute risk reduction there circulates another effect size for binary endpoints in the scientific medical literature: the odds ratio. Relative risk and odds ratio are alternative ways of reflecting study results. Both, relative risk (RR) and odds ratio (OR), can easily be calculated from the "2 x 2-table". Advantage of OR: odds ratios can be calculated in every type of controlled study design, including retrospective studies. Furthermore, odds ratios--the biostatisticians are swarming--offer beautiful mathematical properties and therefore are often used in meta-analysis as an effect size for calculating a pooled estimate of the results of different studies with the same clinical question. Disadvantage of OR: In clinical studies the presentation of the results as "odds ratios" may result in an overestimation of the intervention effect. This article shows the difference between "chance" and "risk" and how odds ratio and relative risk are associated.
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