Modified risk of bias assessment of the 14 included studies using a modified* version of the8 Hoy 2012 tool8
| Study country (Reference) | Representation | Sampling | Random selection | Case definition | Reliability of tool/database validated | Method of data collection * | Prevalence period | Numerators and denominators | Overall risk of bias |
| Australia and New Zealand9 | High | High | Low | Low | High | Low | Low | Low | Moderate |
| Canada11 | High | Low | Low | Low | High | High | Low | Low | Moderate |
| Canada10 | High | High | Low | Low | High | Low | Low | Low | Moderate |
| Finland19 | Low | Low | Low | Low | High | Low | Low | Low | Low |
| Germany14 | High | High | Low | Low | High | High | High | High | High |
| Italy18 | High | High | Low | Low | High | Low | Low | High | Moderate |
| Korea15 | High | High | Low | Low | High | High | Low | Low | Moderate |
| Japan11 | High | High | Low | Low | High | Low | Low | Low | Moderate |
| Poland16 | High | High | Low | Low | High | Low | Low | Low | Moderate |
| Spain17 | High | High | Low | Low | High | Low | Low | High | Moderate |
| Sweden20 | Low | Low | Low | High | High | High | Low | Low | Moderate |
| Switzerland22 | High | High | Low | Low | High | Low | Low | Low | Moderate |
| USA12 | High | High | Low | Low | High | Low | Low | Low | Moderate |
| USA13 | High | High | Low | Low | High | High | High | Low | High |
Excluded the assessment of ‘non-response bias’ and ‘data collection from subjects’ as these are not applicable.
*Retrospective studies were included in high risk.