Expired CME Article

Odds Ratios and Risk Ratios: What’s the Difference and Why Does It Matter?

Authors: Anthony J. Viera, MD, MPH


Odds ratios (OR) are commonly reported in the medical literature as the measure of association between exposure and outcome. However, it is relative risk that people more intuitively understand as a measure of association. Relative risk can be directly determined in a cohort study by calculating a risk ratio (RR). In case-control studies, and in cohort studies in which the outcome occurs in less than 10% of the unexposed population, the OR provides a reasonable approximation of the RR. However, when an outcome is common (iÝ10% in the unexposed group), the OR will exaggerate the RR. One method readers can use to estimate the RR from an OR involves using a simple formula. Readers should also look to see that a confidence interval is provided with any report of an OR or RR. A greater understanding of ORs and RRs allows readers to draw more accurate interpretations of research findings.

Key Points

* When an outcome in a research study is common (eg, occurs in more than 10% of the unexposed group), the odds ratio will tend to overestimate the risk ratio.

* One method to estimate the odds ratio involves using a simple formula.

* A confidence interval should be provided along with any report of an odds ratio or risk ratio.

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