Study population

1. Selection at uniform point C/CC/CS

2. Cases and controls drawn from the same population CC

3. Participation rate > 80% for cases/cohort C/CC

4. Participation rate > 80% for controls CC

Assessment of risk factor

5. Exposure assessment blinded C/CC/CS

6. Exposure measured identically for cases and controls CC

7. Exposure assessed according to validated measures C/CC/CS

Assessment of outcome

8. Outcome assessed identically in studied population C/CC/CS

9. Outcome reproducibly C/CC/CS

10. Outcome assessed according to validated measures C/CC/CS

Study design

11. Prospective design used C/CC

12. Follow-up time > 12 months C

13. Withdrawals < 20% C

Analysis and data presentation

14. Appropriate analysis techniques used C/CC/CS

15. Adjusted for at least age, and gender C/CC/CS

Note. C = applicable to cohort studies; CC = applicable to case-control studies; CS = applicable to cross-sectional studies. Studies were scored as positive (1), negative (0), or unclear (?), and 100% represented the maximum possible score.