3. ROBINS‐I tool (stage 1): prevention.

List of confounding domains relevant to all or most studies (prognostic factors that predict whether an individual receives a SERM/AI versus no SERM/AI)

Age Menopausal status Body mass index Family history of disease Hormone replacement therapy use Benign breast disease Previous cancer other than breast cancer Ethnicity

List of co‐interventions that could differ between intervention groups and could impact on outcome

Hormone replacement therapy Risk‐reducing surgery

SERM/AI: selective oestrogen receptor modulator/aromatase inhibitor