Risk of bias assessment for the prospective studies
| Study | Study participation | Study attrition | Prognostic factor | Outcome measurement | Study confounding | Statistical analysis and reporting |
| Jones and Watson23 | Moderate | Moderate | Moderate | Low | Moderate | Moderate |
| Jones and Macfarlane21 | High | High | High | Low | High | High |
| Sjolie24 | Low | Moderate | High | Low | High | Moderate |
| Szpalski et al 25 | Moderate | High | Moderate | Moderate | Moderate | Moderate |
| Negrini et al 22 2004 | Moderate | High | High | High | High | High |