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.).