| Source of bias | Effect on risk of breast cancer |
| Bias in who takes oestrogen: | Decrease |
| No family history of breast cancer | Decrease |
| Negative mammogram before oestrogen started | Decrease |
| Premature menopause or oophorectomy | Decrease |
| Osteoporosis* | Decrease |
| Higher social class | Increase |
| Thinner* | Increase |
| Late first pregnancy | |
| Bias in diagnosis: | |
| More mammograms and examinations |
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