| Study | Minimizing selection bias1 | Study design1 | Control of confounders1 | Blinding1 | Data collection methods1 | Attrition1 | Overall rating2 |
| Chu 2010 (14) | Moderate | Moderate | Strong | Moderate | Strong | Strong | Strong |
| Gardner 2002 (21) | Strong | Weak | Strong | Moderate | Weak | NA | Weak |
| Horberg 2012 (15) | Strong | Moderate | Strong | Moderate | Strong | NA | Strong |
| Landon 2003 (16) | Moderate | Moderate | Strong | Moderate | Strong | Strong | Strong |
| Landon 2005 (18) | Moderate | Moderate | Strong | Moderate | Strong | Strong | Strong |
| Page 2003 (17) | Moderate | Moderate | Strong | Moderate | Strong | Strong | Strong |
| Schneider 2008 (19) | Moderate | Moderate | Strong | Moderate | Strong | Weak | Moderate |
| Turner 2001 (20) | Moderate | Weak | Strong | Moderate | Strong | Strong | Moderate |
| Wilson 2005 (22) | Moderate | Weak | Strong | Strong | Strong | NA | Moderate |