Table 2.

Assessment of methodological quality according to SIGN Checklists.

Symbol	Criteria
++	High quality, overwhelming majority of criteria fulfilled (>75%), no risk or low risk of bias.
+	Acceptable quality, majority of criteria fulfilled (50–75%), medium risk of bias.
−	Low quality, majority of criteria not fulfilled (<50%), considerable risk of bias.
0	Unacceptable, study rejected due to insufficient quality.