| Study (first author) | Design | Quality score (0–7) | Study population | Sample size | Response | Selection bias | Measurement error | Blinding | Confounding |
| Aittomaki et al 200312 | Cross-sectional | 6 | 1 | 1 | 0 | 1 | 1 | 1 | 1 |
| Eskelinen et al 199113 | Cross-sectional | 4 | 1 | 1 | ? | 0 | 1 | 0 | 1 |
| Fischer et al 200614 | Cross-sectional | 5 | 1 | 0 | 1 | 1 | 0 | 1 | 1 |
| Goedhard et al 199815 | Cross-sectional | 2 | 0 | 1 | ? | ? | 1 | ? | 0 |
| Kaleta et al 200628 | Cross-sectional | 3 | 0 | 1 | 0 | 1 | ? | 1 | 1 |
| Laitinen et al 200516 | Cross-sectional | 5 | 1 | 1 | 0 | 1 | 1 | ? | 1 |
| Laitinen et al 200516 | Prospective cohort | 5 | 1 | 1 | 0 | 1 | 0 | 1 | 1 |
| Martinez et al 200629 | Cross-sectional | 5 | 1 | 1 | 1 | 1 | 0 | 1 | 0 |
| Monteiro et al 200617 | Cross-sectional | 6 | 1 | 1 | 1 | 1 | ? | 1 | 1 |
| Nygard et al 199118 | Cross-sectional | 4 | 1 | 1 | ? | 1 | 1 | 0 | 0 |
| Pohjonen 2001a19 | Prospective cohort | 5 | 1 | 1 | ? | ? | 1 | 1 | 1 |
| Pohjonen 2001b20 | Cross-sectional | 6 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| Pranjic et al 200621 | Cross-sectional | 5 | 1 | 1 | 1 | 1 | 0 | 1 | 0 |
| Punakallio et al 200422 | Cross-sectional | 6 | 1 | 1 | 1 | 1 | 1 | ? | 1 |
| Punakallio et al 200422 | Prospective cohort | 7 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Sjogren-Ronka et al 200223 | Cross-sectional | 6 | 1 | 1 | 1 | 1 | 1 | 0 | 1 |
| Tuomi et al 199124 | Prospective cohort | 5 | 1 | 1 | ? | 1 | 0 | 1 | 1 |
| Tuomi et al 199726 | Prospective cohort | 6 | 1 | 1 | 1 | 1 | 0 | 1 | 1 |
| Tuomi et al 200125 | Cross-sectional | 5 | 1 | 1 | 1 | 0 | 0 | 1 | 1 |
| Tuomi et al 200427 | Prospective cohort | 5 | 1 | 1 | 0 | 1 | 0 | 1 | 1 |
1, “sufficiently met”; 0, “not sufficiently met”; ‘?’, information was lacking.