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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 |
