Study quality domains by study design using the Newcastle-Ottawa Scale
| High quality (%) (score >6) | Medium quality (%) (score 5–6) | Low quality (%) (score <5) | Mean score (maximum 9) | |
| Study design | ||||
| All (n=17) | 76.5 | 17.6 | 5.9 | 6.9 (1.2) |
| Cohort (n=11) | 81.8 | 9.1 | 9.1 | 7.2 (0.9) |
| Case-control (n=6) | 66.7 | 16.7 | 16.7 | 6.3 (1.5) |
| Selection (maximum 4) | Comparability (maximum 2) | Outcome or exposure (maximum 3) | Mean total score (maximum 9) | |
| Study design | ||||
| Cohort | 2.6 (0.7) | 2 (0) | 2.5 (0.7) | 7.2 (0.9) |
| Case-control | 2.5 (1.2) | 1.8 (0.4) | 2 (0) | 6.3 (1.5) |
| Year of publication | ||||
| 1999 or earlier | 2 (0) | 2 (0) | 3 (0) | 7 (0) |
| 2000–2009 | 2.25 (0.5) | 2 (0) | 2.5 (0.6) | 6.8 (0.5) |
| 2010 or later | 2.8 (1.0) | 1.9 (0.3) | 2.2 (0.6) | 6.9 (1.4) |
The mean with SD in brackets is shown where applicable.