When investigating the association between HRT use and breast cancer risk, it is, therefore, necessary to adjust for the potential confounder, ‘age at menopause’. Using data on 13 357 postmenopausal women in whom 396 cases of invasive breast cancer were diagnosed during nine years of follow-up from the Melbourne Collaborative Cohort Study, we compared the estimates of the relative risk of HRT use for breast cancer for three different methods of dealing with missing data: complete-case analysis, single imputation, and multiple imputations.
‘Age at menopause’ was missing for 17% of the data. Both HRT use and ‘age at menopause’ were significant risk factors for breast cancer, although ‘age at menopause’ only marginally confounded the estimates of risk for HRT. Complete-case analyses resulted in higher estimates of the risk associated with HRT use than the different imputation methods.
The study concluded that analyses investigating the association between HRT and breast cancer should present the results in two ways: excluding women with ‘age at menopause’ missing and excluding women using multiple imputations. For both methods, estimates of risk, with and without the adjustment of ‘age at menopause’, should be given.