Rate ratios are the ratio of the incidence rate in an exposed group divided by the incidence rate in an unexposed (or less exposed) comparison group.
Consider an example from The Nurses’ Health Study. This prospective cohort study was used to investigate the effects of hormone replacement therapy (HRT) on coronary artery disease in postmenopausal women. The investigators calculated the incidence rate of coronary artery disease in postmenopausal women who had been taking HRT and compared it to the incidence rate in post-menopausal women who had not taken HRT. The findings are summarized in this table:
| Post-menopausal Hormone Use | # with Coronary Artery Disease | Person-Years of Disease-free Follow-up |
| Yes | 30 | 54,308.7 |
| No | 60 | 51,477.5 |
- The rate in those using hormones was 30 / 54,308.7 = 55.2 per 100,000 person-years
- The rate in those NOT using hormones was 60 / 51,477.5 = 116.6 per 100,000 person-years.
So, the rate ratio was 0.47.
Interpretation: Women who used postmenopausal hormones had 0.47 times the rate of coronary artery disease compared to women who did not use postmenopausal hormones.
(Rate ratios are often interpreted as if they were risk ratios, e.g., post-menopausal women using HRT had 0.47 times the risk of CAD compared to women not using HRT, but it is more precise to refer to the ratio of rates rather than risk.)