Bias in breast cancer analyses due to error in age at menopause

With the use of data from the Nurses' Health Study, the authors confirm the recent argument by Pike et al. (Am J Epidemiol 1998;147:718-21) that vario us ways of assigning ages at menopause to women with simple hysterectomy pr oduce relative risks for the effect of hormone replacement therapy (HRT) th at are biased downward. Different methods of including women with simple hy sterectomy consistently produced relative risks for the association between a 1-year increase in duration of HRT use and breast cancer that were under estimates compared with the relative risk obtained when these women were ex cluded from analysis. The authors used longitudinal data on recalled age at menopause to demonstrate another source of error related to age at menopau se: error in recall of age at menopause that increases with time since meno pause.