Tuppurainen 1994.

Methods	Kuopio Osteoporosis Risk Factor and Prevention Study; cohort study began in 1989 Postal questionnaires to all women born 1932 to 1941 and living in Kuopio, Finland (N = 14,220; response N = 13,100) Questions included health‐related factors, comorbidity, medications, and anthropometrics.
Participants	Random sample (from baseline survey respondents) of women willing to undergo bone densitometry (dual‐energy X‐ray absorptiometry (DXA); 11,055 women willing (of 13,100 women who responded to initial questionnaire) Stratified random sample selected to participate in bone densitometry (N = 3686); 3222 women had bone mineral density (BMD) assessed in 1990 to 1991; 2942 women had valid spine measurements and 3203 women had valid hip measurements. Exclusion criteria: hip deformity (N = 19), spine osteophytes or deformities (N = 280)
Interventions	Oral contraceptive use (sum of progestin‐only and combination OC use)
Outcomes	Fracture history
Notes	Additional study methodology was obtained from later article (Sirola 2012). Website had basic information on study design. Unable to obtain further information from investigator regarding questionnaire items for OC use and fractures. Investigator communicated that women were followed for 20 years and additional data might be provided.
Risk of bias
Bias	Authors' judgement	Support for judgement
Case definition and representativeness	Unclear risk	Not applicable
Exposed cohort: representativeness	Low risk	Exposed cohort: (N = 939) women who had ever used OCs (perimenopausal women willing to have bone density assessed)
Control selection and definition; non‐exposed cohort selection	Low risk	Non‐exposed cohort: (N = 2283) women who had never used OCs (perimenopausal women willing to have bone density assessed)
Exposure ascertainment, including same method for cases and controls	Unclear risk	Duration and purpose of lifetime OC use; sum of progestin‐only and COC use Data from initial mail survey; no follow‐up questions when DXA done
Outcome assessment (cohort study): method and evidence outcome not present at study start	High risk	Fracture history came from initial survey (retrospective data); no follow‐up questions when DXA done and no external validation No distinction among types of fractures in report Fractures calculated as (1) all fractures since age 15 until BMD measurement and (2) all fractures during 1980s
Comparability of groups on basis of design or analysis	High risk	Differences between groups examined in univariate analyses. OC group differed from non‐OC group in age, time since menopause, parity, smoking, marital status, education, urban‐rural living. Fracture rates not adjusted for confounding Earlier paper (1993) examined fractures in 1985 to 1989; multivariate logistic regression included OC use > 6 years plus potential confounders such as age, weight and height, and parity. No detail on model development
Case‐control: non‐response rate	Unclear risk	Not applicable
Cohort: follow‐up length and adequacy (including loss to follow‐up)	High risk	Response for survey 92% (13,100/14,220); for DXA 87% (3222/3686). Study apparently conducted with data from first contact; no follow‐up; losses not applicable. Osteoporosis study continued at least 15 years, according to later articles.