Quality assessment of studies included in the systematic review
| Study | Selection bias | Exposure bias | Outcome assessment bias | Confounding factor bias | Analytical bias | Attrition bias | Overall likelihood of bias |
| Black et al23 | Low | Minimal | Minimal | Low | Minimal | Moderate | Low |
| Gendelman et al24 | Moderate | Minimal | Minimal | Moderate | Moderate | Low | Moderate |
| Lin et al25 | Minimal | Minimal | Minimal | Minimal | Minimal | Low | Minimal |
| Pearson et al26 | Low | Low | Minimal | Moderate | Moderate | Moderate | Moderate |
| Salmi et al27 | Minimal | Low | Low | Not reported | Not reported | Minimal | Moderate |
| Williams et al13 | Low | Low | Low | Minimal | Low | Low | Low |
| Winkley et al28 | Low | Minimal | Minimal | Minimal | Low | Minimal | Minimal |
The six different types of bias were classified as ‘minimal’, ‘low’, ‘moderate’, ‘high’ or ‘not reported’ based on criteria outlined previously in O’Neill et al.22