Chang 1999.

Methods	Case‐control study
Participants	291 cases/736 controls Aged 20 to 44 years
Interventions	Combined oral contraception: current and none use, < 50 µg of oestrogen and ≥ 50 µg of oestrogen
Outcomes	Ischemic stroke Events: current use 19/42, none use 41/146, <50 µg 10/28, ≥ 50 µg 9/14
Notes	UK, Germany, Hungary, Serbia and Slovenia
Risk of bias
Bias	Authors' judgement	Support for judgement
Exposure ascertainment	High risk	Ischemic stroke not objectively confirmed.
Outcome assessment	Low risk	COC use 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.