Quality assessment: Newcastle-Ottawa Scale
| Study, year | Number of stars | |||
| Selection* | Comparability† | Exposure‡ | Overall | |
| Almirall et al,45 1999 | 4 | 1 | 1 | 6/9 |
| Almirall et al,44 2008 | 3 | 1 | 1 | 5/9 |
| Baik et al,10 2000 | 4 | 2 | 2 | 8/9 |
| Breitling et al,39 2016 | 3 | 2 | 2 | 5/9 |
| Clough et al,42 2003 | 4 | 0 | 1 | 5/9 |
| Fernández-Solá et al,43 1995 | 3 | 2 | 1 | 6/9 |
| Inoue et al,36 2007 | 3 | 1 | 1 | 5/9 |
| Jackson et al,46 2009 | 4 | 1 | 1 | 6/9 |
| Koivula et al,37 1994 | 4 | 1 | 3 | 8/9 |
| Lipsky et al,111986 | 3 | 0 | 2 | 5/9 |
| Loeb et al,48 2009 | 2 | 2 | 1 | 5/9 |
| Phung and Wang,38 2013 | 3 | 0 | 3 | 6/9 |
| Quraishi et al,49 2013 | 1 | 0 | 1 | 2/6 |
| Shen et al,35 2013 | 3 | 2 | 3 | 8/9 |
| Watt et al,47 2007 | 4 | 2 | 1 | 7/9 |
| Yende et al,40 2013 | 4 | 0 | 2 | 6/9 |
| Zaridze et al,41 2009 | 3 | 2 | 1 | 6/9 |
*Maximum 4 stars.
†Maximum 2 stars.
‡Maximum 3 stars.