Shapiro 1979.

Methods	Case‐control study
Participants	234 cases/1742 controls Aged 25 to 49 years
Interventions	Combined oral contraception: < 50 µg oestrogen, 50 µg oestrogen, > 50 µg oestrogen
Outcomes	Myocardial infarction Events: < 50 µg oestrogen 8/39, 50 µg oestrogen 16/78, > 50 µg oestrogen 2/11
Notes	USA
Risk of bias
Bias	Authors' judgement	Support for judgement
Exposure ascertainment	High risk	COC use not objectively confirmed.
Outcome assessment	Low risk	Myocardial infarction objectively confirmed.
Follow‐up	Unclear risk	Not applicable as this was a case‐control study.
Source population	High risk	Controls from a different source population to the cases.