Ratings of methodological quality: strong, moderate and weak
| Study | Selection bias | Study design | Confounders | Blinding | Data collection | Withdrawals | Global rating |
| Thewes et al, 200931 | Moderate | Moderate | Weak | Moderate | Weak | Moderate | Weak |
| Braeken et al, 2009,26 201327 and 201328 | Moderate | Strong | Strong | Moderate | Weak | Strong | Moderate |
| Ito et al, 201125 | Moderate | Moderate | Strong | Moderate | Weak | Moderate | Moderate |
| Zemlin et al, 201129 | Moderate | Moderate | Weak | Moderate | Weak | Moderate | Weak |
| Bauwens et al, 201430 | Moderate | Moderate | Weak | Weak | Weak | Weak | Weak |