Quality assessment/risk of bias of observational studies included in the review
| First author [ref.] | Selection bias | Confounding | Measurement of outcome | Missing data | Reporting bias |
| Broomhead [28] | Unclear | High | High | Low | Unclear |
| Chuck [22] | Unclear | High | High | Low | Unclear |
| Wade [23] | Unclear | High | High | Unclear | Unclear |