The table above illustrates the distribution of the Factor V Leiden mutation (S+) and exposure to OCP (E+) in the control population.

To determine the absolute risk (incidence) of VT among women in the 4 groups, the authors used the known incidence rate of VT in the population (2.1/10,000/yr) to determine the total number of person years required for the 155 observed cases of VT to develop. This number was ~740,000 person years, and they were distributed among the 4 groups according to the prevalence rates shown above. For example, 740,000 x 0.012 = ~8880 person years required for the 25 cases in the S+E+ group (shown in slide 22) to have occurred. Therefore, the incidence of VT among women who had the Factor V Leiden mutation and were exposed to OCP could be estimated as 25 / 8,880 x 10,000 = 28/10,000 per year. These calculations were repeated for each of the groups and are illustrated on slide 26.