At the end of last week’s look at HRT and breast cancer, I promised that I would look at “The Bradford Hill criteria” this week. The thing that prompted me to do this was that the headlines in the HRT/breast cancer study were “Risk of breast cancer nearly tripled...” The study claimed that the risk was 2-3 times higher for women taking the oestrogen/progesterone form of HRT relative to women not taking HRT at all.
That study was the first that I can remember dissecting in years where the relative risk ratio was 2-3 times. Differences are usually 20-40%, or similarly small – not 2-3 times. Take the recent exercise study – I dissected this one to show that if you did more than 13 times the recommended amount of exercise (13 times!) vs. less than even the recommended amount, you had a claimed 28% reduced risk of diabetes.
Two to three times is thus unusual and it meets one of nine criteria, which should be known far more widely than they are and used far more often than they are. Welcome to a brilliant test of association studies, by a brilliant emeritus professor of medical statistics...
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