To the Editor: We question the conclusions drawn by Canfell and colleagues1 from their analysis of trends in hormone replacement therapy (HRT) prevalence and breast cancer incidence for Australian women aged 50 years or older. Their ecological analysis lacks individual-level information on HRT use and information on tumour oestrogen receptor (ER) status, and captures only 2 years following the decline in HRT prevalence. This is an inadequate design within which to judge issues of causality; it is at best an hypothesis-generating exercise.2
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