Table 1

Best-evidence synthesis used in this review8

StrongGenerally consistent findings were presented in multiple high-quality cohort studies
ModerateOne high-quality cohort study and at least two high-quality case–control studies, or when at least three high-quality case–control studies show generally consistent findings
LimitedGenerally consistent findings were found in a single cohort study, or in maximum two case–control studies, or in multiple cross-sectional studies
ConflictingLess than 75% of the studies reported consistent findings
No evidenceNo study could be found