Table 2. Quality assessment of included case-control studies using the Newcastle-Ottawa quality assessment scale.
| Study | Case defined with independent validation | Representativeness of the cases | Selection of controls from community | Statement that controls have no history of outcome | Cases and controls matched and/or adjusted by factors | Ascertain exposure by secure record/blinded structured interview | Same method of ascertainment for cases and controls | Same response rate for both groups | Overall Score |
|---|---|---|---|---|---|---|---|---|---|
| Amaducci, 1986 | 1 | 1 | 0 | 1 | 2 | 1 | 1 | 1 | 8 |
| Li, 1992 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 9 |
| CSHA, 1994 | 1 | 1 | 1 | 0 | 2 | 0 | 1 | 1 | 7 |
| Bohnen, 1994 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 9 |
| Gasparini, 2002 | 1 | 1 | 0 | 0 | 2 | 1 | 1 | 1 | 7 |
| Harmanci, 2003 | 1 | 1 | 1 | 0 | 2 | 1 | 1 | 1 | 8 |
| Zuo, 2010 | 1 | 1 | 0 | 0 | 2 | 1 | 1 | 1 | 7 |
| Sprung, 2013 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 9 |
| Bufill, 2009 | 1 | 1 | 1 | 0 | 2 | 1 | 1 | 1 | 8 |
| Chen, 2014 | 1 | 1 | 1 | 0 | 2 | 1 | 1 | 1 | 8 |
| Tsuda, 2015 | 1 | 1 | 0 | 0 | 0 | 1 | 1 | 1 | 5 |
| Breteler, 1991-ref2 | 1 | 0 | 1 | 0 | 2 | 1 | 1 | 1 | 7 |
| Breteler, 1991-ref7 | 1 | 1 | 0 | 1 | 2 | 1 | 1 | 1 | 8 |
| Breteler, 1991-ref8 | 1 | 1 | 0 | 0 | 2 | 1 | 1 | 1 | 7 |
| Breteler, 1991-ref9 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 9 |
| Breteler, 1991-ref31 | 1 | 1 | 1 | 0 | 2 | 1 | 1 | 1 | 8 |
| Breteler, 1991-ref32 | 1 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 9 |