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Quality assessment conducted according to the Newcastle-Ottawa Scale for all the included studies
| First author | Quality indicators | ||
|---|---|---|---|
| Selection | Comparability | Exposure | |
| Bergink 2003 | *** | * | ** |
| Jin 2004 | *** | * | *** |
| Xue 2004 | *** | * | *** |
| Tian 2009 | ** | * | ** |
| Yang 2009 | *** | * | ** |
| Riancho a 2010 | *** | * | ** |
| Riancho b 2010 | *** | * | ** |
| Riancho c 2010 | *** | * | *** |
| Borgonio-Cuadra 2012 | ** | * | *** |
| Dai 2014 | *** | * | ** |
| Liu 2014 | **** | * | ** |
| Jiao 2007 | ** | * | ** |
| Tawonsawatruk 2009 | *** | * | ** |
