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.”