Bone Mass and the Risk of Breast Cancer

To the Editor: Zhang et al. (Feb. 27 issue) 1 present data confirming the finding of the Study of Osteoporotic Fractures Research Group 2 that increased bone mass in postmenopausal women is associated with an increased risk of subsequent breast cancer. However, I am concerned about the accuracy of the multivariate-adjusted rate ratios that Zhang et al. report. The authors included 10 variables in addition to bone mass in their multivariate model. Since the number of observed events (i.e., cases of breast cancer) was only 91, the ratio of the number of events to the number of variables is less than 10. . . .