Odds ratios and risk ratios: what's the difference and why does it matter?

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 (iY 10% 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.