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Risk ratios, odds ratios, and hazard ratios are three common, but often misused, statistical measures in clinical research. Understanding the three measures’ applicability and usage allows for a more accurate interpretation of study results and a better understanding of what each value demonstrates and, equally importantly, what each does not.

- George et al.: What's the Risk: Differentiating Risk Ratios, Odds Ratios, and Hazard Ratios?

Odds ratio (OR) is a statistic commonly encountered in professional or scientific medical literature. Most readers perceive it as relative risk (RR), although most of them do not know why that would be true. Another statistic, which is often also perceived as a relative risk, is the hazard ratio (HR).

Either the OR or RR could be used in many study types. However, only the OR can be used in case-control studies. Because in order to calculate the RR, one must know the risk. Risk is a probability, a proportion of those exposed with an outcome compared to the total population exposed. This is impossible in a case-control study, in which those who already have the outcome are included without knowing the total population exposed. Read more from Clay Smith's Idiot's Guide to Odds Ratios.