Quality assessment
The quality of the included studies was evaluated independently by two
authors using the Newcastle-Ottawa Quality Assessment Scale (NOS). The
NOS scale includes three domains, the maximum scores are four points for
selection(representativeness of the exposed cohorts, selection of the
non-exposed cohorts, ascertainment of exposure, and demonstration of the
absence of an outcome of interest at study initiation), and two points
for comparability (comparability of cohorts based on the design or
analysis), and three for outcome assessment (assessment of outcome,
follow-up long enough to produce results and adequacy of cohort
follow-up). One point is awarded for each item that is satisfied by the
study, except for the second part of the scale (comparability of Cohorts
on the Basis of the Design or Analysis), which is capped at two points.
The scale is scored out of 9, with a cumulative score of
>=6 being a high-quality study and less than 6 stars being
a low-quality study. The study was evaluated by two investigators, and a
third investigator was consulted if there was disagreement.