Hypothetical example of how the odds ratio is an unbiased estimate of the incidence rate ratio in an incidence density case-control study. The sampling fraction for cases is 10% and the control group provides the estimate of the fraction of person-time exposed and unexposed. This example is based on data from Thompson et al 4
| Hypothetical cohort | Helmet | No helmet | Total | Incidence rate ratio |
|---|---|---|---|---|
| Cases | 2 220 | 5 350 | 7 570 | (2220/149554)/(5350/113746) = 0.316 |
| Bicycle riding hours | 149 554 | 113 746 | 263 300 | |
| Incidence density sampling from the population experience: | ||||
| Odds ratio | ||||
| Cases | 222 | 535 | 757 | (222/1496)/(535/1138) = 0.316 |
| Controls | 1 496 | 1 138 | 2 634 | |