Quality assessment
The quality of the included studies was assessed using the Newcastle-Ottawa scale (NOS) (15). This assessment tool is recommended by the Cochrane collaboration to assess risk of bias of non randomised observational studies. The tool uses three domains: selection of study groups (four points); comparability of groups (two points); and ascertainment of exposure and outcomes (three points). The higher the score the less is the risk of bias. The maximum score on the Newcastle-Ottawa scale is 9. Thresholds for converting the Newcastle-Ottawa scales to AHRQ standards (good, fair, and poor):
“-Good quality: three or four stars in selection domain + one or two stars in comparability domain + two or three stars in outcome/exposure domain
-Fair quality: two stars in selection domain + one or two stars in comparability domain + two or three stars in outcome/exposure domain.
-Poor quality: zero or one star in selection domain OR zero stars in comparability domain OR zero or 1 stars in outcome/exposure domain.”