Quality assessment and the risk of bias
The risk of bias and the quality of the observational studies were
assessed using the Newcastle-Ottawa Quality Assessment Scale (NOS). The
scale is based on eight criteria and provides a score ranging from 0
(high risk of bias) to 9 stars (low risk of bias). A 5-star rating and
below was designated high risk of bias, 6-7 stars intermediate risk of
bias, and 8-9 stars low risk of bias. Randomized controlled studies were
evaluated by the Cochrane Collaboration’s Risk of Bias Tool. Summary
assessments of the risk of bias were derived for each study. Assessments
were carried out independently by two investigators (A.H and N.T.).