Study quality: reporting and risk of bias summary (Downs and Black27)
| Study lead author, date | Reporting score n/11 | External validity score n/3 | Internal validity—bias score n/7 | Internal validity—confounding score n/6 | Power score n/1 | Total score n/28 | Quality level* |
|---|---|---|---|---|---|---|---|
| Ferrell, 1995 | 4 | 1 | 2 | 0 | 0 | 7 | Poor |
| Wells, 2003 | 6 | 1 | 4 | 2 | 0 | 13 | Poor |
| Keefe, 2005 | 9 | 3 | 3 | 5 | 0 | 20 | Good |
| Lin, 2006 | 6 | 1 | 3 | 2 | 0 | 12 | Poor |
| Ward, 2009 | 8 | 2 | 5 | 4 | 1 | 20 | Good |
| Capewell, 2010 | 6 | 1 | 2 | 1 | 0 | 10 | Poor |
| Vallerand, 2010 | 5 | 1 | 3 | 2 | 0 | 11 | Poor |
| Valeberg, 2013 | 6 | 1 | 3 | 2 | 0 | 12 | Poor |
*Based on Samoocha et al's28 classification of quality level: excellent (26–28); good (20–25); fair (15–19); poor (≤14).