Methodological quality of included studies (the Newcastle Ottawa Scale (NOS) adapted version)
| First author | Year | Selection bias | Performance bias | Detection bias | Information bias | Total score | |||
| A | B | C | D | E | F | G | |||
| Badcock et al 52 | 2002 | 1 | 0 | 2 | 1 | 0 | 2 | 2 | 8/21 |
| Chester et al 22 | 2016 | 3 | 3 | 2 | 2 | 0 | 0 | 2 | 12/21 |
| Cho et al 39 | 2015 | 2 | 3 | 0 | 1 | 0 | 2 | 2 | 10/21 |
| Cho et al 40 | 2017 | 2 | 3 | 0 | 1 | 0 | 2 | 2 | 10/21 |
| Coronado et al 53 | 2017 | 1 | 0 | 2 | 2 | 2 | 3 | 2 | 12/21 |
| Dekker et al 41 | 2016 | 2 | 3 | 0 | 1 | 0 | 2 | 2 | 10/21 |
| Ekeberg et al 51 | 2010 | 1 | 0 | 2 | 2 | 3 | 2 | 2 | 12/21 |
| Engebretsen et al 54 | 2010 | 1 | 0 | 3 | 2 | 3 | 2 | 2 | 13/21 |
| George et al 36 | 2008 | 1 | 0 | 1 | 2 | 0 | 2 | 2 | 8/21 |
| Gill et al 23 | 2013 | 1 | 0 | 3 | 2 | 3 | 2 | 2 | 13/21 |
| Henn III et al 20 | 2007 | 1 | 0 | 2 | 2 | 3 | 2 | 2 | 12/21 |
| Jawa et al 42 | 2016 | 1 | 0 | 0 | 0 | 3 | 1 | 2 | 7/21 |
| Karlsson et al 55 | 2016 | 0 | 0 | 2 | 2 | 3 | 3 | 2 | 12/21 |
| Koorevaar et al 43 | 2016 | 1 | 0 | 2 | 2 | 3 | 3 | 2 | 13/21 |
| Kromer et al 24 | 2014 | 1 | 0 | 2 | 2 | 3 | 3 | 2 | 13/21 |
| Macfarlane et al 56 | 1998 | 1 | 0 | 2 | 0 | 0 | 2 | 2 | 7/21 |
| Oh et al 21 | 2012 | 1 | 0 | 0 | 1 | 3 | 2 | 2 | 9/21 |
| Potter et al 44 | 2015 | 1 | 0 | 0 | 2 | 0 | 3 | 2 | 8/21 |
| Razmjou et al 45 | 2011 | 2 | 1 | 1 | 2 | 1 | 3 | 2 | 12/21 |
| Reilingh et al 25 | 2008 | 1 | 0 | 3 | 2 | 0 | 2 | 2 | 10/21 |
| Styron et al 46 | 2015 | 1 | 0 | 2 | 1 | 1 | 1 | 2 | 8/21 |
| Tokish et al 47 | 2017 | 1 | 0 | 0 | 0 | 3 | 1 | 2 | 7/21 |
| Valencia et al 37 | 2011 | 1 | 0 | 0 | 1 | 0 | 3 | 2 | 7/21 |
| Valencia et al 38 | 2014 | 1 | 0 | 2 | 2 | 3 | 3 | 2 | 13/21 |
| Werner et al 48 | 2016 | 1 | 0 | 1 | 1 | 3 | 1 | 2 | 9/21 |
| Werner et al 49 | 2017 | 2 | 3 | 1 | 1 | 3 | 1 | 2 | 13/21 |
| Yeoman et al 50 | 2012 | 2 | 3 | 0 | 0 | 3 | 3 | 2 | 13/21 |
A, Is the source population (cases, controls, cohorts) appropriate and representative of the population of interest? B, Is the sample size adequate and is there sufficient power to detect a meaningful difference in the outcome of interest? C, Did the study identify and adjust for any variables or confounders that may influence the outcome? D, Did the study use appropriate statistical analysis methods relative to the outcome of interest? E, Is there little missing data and did the study handle it accordingly? F, Is the methodology of the outcome measurement explicitly stated and is it appropriate? G, Is there an objective assessment of the outcome of interest?