Abstract
Case-control and cohort studies are almost always complicated by nonrandom exposure allocation, which must be minimized in the design and analysis phases. Tubal sterilization is a common gynecological procedure that may be associated with other reproductive organ surgeries, which in turn may be associated with breast cancer risk. In this issue of the Journal, Gaudet et al. (Am J Epidemiol. 2013;177(6):492-499) argue successfully that tubal sterilization is unassociated with breast cancer risk. Scrutiny of the heterogeneous studies included in their meta-analysis underscores the role of confounding and effect modification in observational epidemiologic studies. Specifically, tubal sterilization is unassociated with breast cancer risk, but either oophorectomy or hysterectomy, or both, and the timing of these procedures warrant careful consideration in the design, analysis, and interpretation of observational research on reproductive factors.