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Quality assessment of the case–control studies according to the Newcastle-Ottawa scale
| Literature | Selection of enrolled study subjects | Between-group comparability | Exposure outcomes and factors | Total |
|---|---|---|---|---|
| Kumawat (2018-India) | 2 | 3 | 3 | 8 |
| Bethke (2008-UK-Nouth) | 3 | 3 | 3 | 9 |
| Bethke (2008-UK-Southeast) | 2 | 3 | 3 | 8 |
| Bethke (2008-Sweden) | 2 | 3 | 3 | 8 |
| Bethke (2008-Denmark) | 2 | 3 | 3 | 8 |
| Bethke (2008-Finland) | 2 | 3 | 3 | 8 |
| Semmler (2006-Germany) | 3 | 3 | 3 | 9 |
| Average | 2.3 | 3.0 | 3.0 | 8.3 |
