| Authors† | Design and material | Adjusted for confounders | Measurement of outcome | Measurement of exposure | Data presentation, statistical analysis | Quality assessment‡ total score | |
| Lindberg, 198437 | 1 | 0 | 1 | 1 | 0 | 3 | + |
| Typpö T, 198524 | 2 | 0 | 1 | 1 | 1 | 5 | + |
| Jacobsen, 198720 | 1 | 0 | 0 | 1 | 1 | 3 | + |
| Thelin, 199034 | 2 | 1 | 3 | 2 | 2 | 10 | ++ |
| Vingaard, 199127 | 2 | 1 | 1 | 2 | 2 | 8 | ++ |
| Vingaard, 199122 25 | 3 | 3 | 2 | 2 | 3 | 13 | +++ |
| Vingaard, 199226 | 1 | 0 | 1 | 2 | 2 | 6 | ++ |
| Croft, 199216 | 2 | 3 | 3 | 2 | 3 | 13 | +++ |
| Croft, 199231 | 2 | 2 | 2 | 1 | 3 | 10 | +++ |
| Axmacher, 199330 | 2 | 1 | 0 | 2 | 0 | 5 | + |
| Roach, 199423 | 2 | 2 | 1 | 2 | 2 | 9 | ++ |
| Jensen, 199432 | 2 | 1 | 2 | 1 | 2 | 8 | ++ |
| Vingaard, 199728 | 3 | 3 | 3 | 2 | 3 | 14 | +++ |
| Thelin, 199733 | 3 | 2 | 2 | 2 | 2 | 11 | +++ |
| Coggon, 199815§ | 2 | 2 | 3 | 2 | 3 | 12 | +++ |
| Cviteticl, 199917 | 2 | 2 | 0 | 1 | 2 | 7 | ++ |
| Yoshimura, 200029 | 2 | 2 | 2 | 1 | 2 | 9 | ++ |
| Lau, 200021 | 1 | 1 | 2 | 2 | 2 | 8 | ++ |
| Flugsrud, 200218 | 2 | 2 | 3 | 1 | 3 | 11 | +++ |
| Tüchsen, 200336 | 3 | 1 | 2 | 2 | 3 | 11 | +++ |
| Thelin, 200435 | 2 | 2 | 2 | 2 | 2 | 10 | ++ |
| Jacobsen, 200438 | 1 | 0 | 3 | 3 | 0 | 7 | ++ |
*Each item was scored from 1–3. Quality of the study was assessed by the sum of the scores, the maximum being 15 points.
†Several of the studies included more than one exposure, and have been cited more than once.
‡The papers were subdivided into “+” poor quality (score 1–5); “++” medium quality (score 6–10); “+++” high quality (score 11–15).
§The study only scored 5 points (+) in relation to the study of farmers.