EDITOR—Farmer et al cited several “crucial differences” between a study they published and our recent study on the risk of venous thromboembolism associated with third generation oral contraceptives compared with oral contraceptives containing levonorgestrel using the same data source, the General Practice Research Database.1-3 We agree that it is worth while to consider how the differences Farmer et al allude to might affect the findings of our studies.

Firstly, Farmer et al point out that over the years of our using the database to conduct pharmacoepidemiological research at the Boston Collaborative Drug Surveillance Program we have restricted our studies to a subset of practices that have provided reliably high quality data.1 They imply that this may introduce selection bias (at the level of general practice rather than individual subjects). This is not, however, a plausible explanation for the difference between our findings. If there were truly no difference in the risk of venous thromboembolism with third generation …