Response to Yang and Chan. Metformin and the Risk of Cancer: Time-Related Biases in Observational Studies. Diabetes Care 2012;35:2665–2673

In the comment by Bodmer et al. (1), the authors question our assertion that several of their studies suffered from time-window bias (2). Perhaps there are subtleties about this bias that need to be further clarified.In the authors’ breast cancer study (3), cases and controls were matched on age, sex (although all females), general practice, and index date (i.e., the calendar date of the breast cancer diagnosis). However, there was no matching on time since diabetes diagnosis or since the first prescription for an oral hypoglycemic agent. Not matching on this entry point can result in artificially different exposure patterns between cases and controls (a patient with 5 years’ diabetes …