Table 1

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 cohortHelmetNo helmetTotalIncidence rate ratio
Cases2 2205 3507 570(2220/149554)/(5350/113746) = 0.316
Bicycle riding hours149 554113 746263 300
Incidence density sampling from the population experience:
Odds ratio
    Cases222535757(222/1496)/(535/1138) = 0.316
    Controls1 4961 1382 634