1. Evidence quality assessment, case‐control studies.

Studya	Selection				Comparability of cases and controls	Exposure			Evidence qualityb
	Case definition	Cases representative	Control selection	Control definition		Ascertainment method	Same ascertainment both groups	Nonresponse rate
La Vecchia 1999	✸	✸	‐	‐	✸	‐	✸	✸	Very low
Mallmin 1994	✸	✸	✸	✸	✸	‐	✸	✸	Low
Meier 2010	✸	✸	✸	‐	✸✸	✸	✸	✸	Moderate
Memon 2011	✸	✸	✸	✸	✸✸	✸	✸	✸	High
Michaelsson 1999	✸	✸	✸	‐	✸✸	‐	✸	✸	Low
O'Neill 1996	✸	✸	✸	‐	✸✸	‐	✸	‐	Low
Vestergaard 2006c	✸	✸	✸	‐	✸✸	✸	✸	✸	Moderate

^aNewcastle‐Ottawa Quality Assessment Scale (Appendix 3): 1 star (✸) for meeting each criterion, except comparability (design or analysis) can have 2 stars. For comparability in this review: 1 star if controlled for age; 2 stars if also controlled for other important variables, e.g., exercise, body mass index, use of hormone replacement therapy or other relevant drugs
 ^bModerate quality evidence: met criteria for selection (4 items), comparability (1 star; upgraded for 2 stars), and ascertainment method; downgrading due to design limitation or lack of information in report
 ^cVestergaard 2006 includes 3 reports: OC use (2006), OC use among young women (2008a); use of DMPA or hormonal IUD (2008b)