Quality assessment
The quality of included studies and risk of bias were assessed by two
reviewers using Newcastle Ottawa Scale (NOS, a design-specific scale
–case–control vs. cohort study) independently (14). Three items were
contained in NOS, including selection of study subjects, comparability,
exposure assessment and outcome. Eight items were used to identify the
quality of study. In exposure/outcome assessment, each item could be
given one star, and comparability a maximum of two stars. As such, the
total score ranges from zero to nine stars (15). Study inclusion was not
affected by quality score, but considered in sensitivity analysis and
results interpretation.