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

South Med J. 2008 Jul;101(7):730-4. doi: 10.1097/SMJ.0b013e31817a7ee4.


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.

MeSH terms

  • Case-Control Studies
  • Cohort Studies
  • Humans
  • Odds Ratio*
  • Risk*