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Quality assessment according to the Newcastle–Ottawa scale.
| Study | Selection | Comparability | Exposure | Total score |
|---|---|---|---|---|
| Richard 2011 | 3 | 2 | 2 | 7 |
| Jorge-Mora 2012 | 4 | 0 | 2 | 6 |
| Schmelzer-Schmied 2009 | 3 | 2 | 2 | 7 |
| Rizzo 2008 | 3 | 2 | 2 | 7 |
| Wright 2005 | 3 | 0 | 2 | 5 |
| Westhal 2005 | 3 | 2 | 2 | 7 |
| Kumbaraci 2014 | 3 | 2 | 2 | 7 |
| Gereli 2010 | 2 | 0 | 2 | 4 |
| Sato 2010 | 3 | 2 | 2 | 7 |
| Zhao 2011 | 3 | 2 | 2 | 7 |
| Yang 2013 | 2 | 2 | 2 | 6 |
