Table 2: Methodological appraisal of the included studies.

Included studies (n= 33) Study participation Study attrition Prognostic factor measurement Outcome measurement Confounding measurement and account Analysis Total score
/12
Alzharani et al. (2011) [33] 2 1 2 2 2 2 11
Ayers et al. (2005) [42] 2 1 2 2 0 2 9
Baker et al. (2012) [16] 1 0 2 2 0 2 7
Caracciolo et al. (2005) [37] 2 1 2 2 0 2 9
Clement et al. (2013) [35] 1 1 2 2 0 2 8
Clement et al. (2013) [32] 2 2 2 2 0 2 10
Clement et al. (2013) [40] 1 1 2 2 0 2 8
Davis et al. (2008) [34] 2 2 2 2 0 2 10
Desmeules et al. (2013) [17] 2 2 2 2 2 2 12
Engel et al. (2004) [51] 1 2 2 2 0 2 9
Escobar et al. (2007) [24] 2 0 2 2 2 2 10
Fortin et al. (1999)9 & (2002) [15] 2 0 2 2 0 2 8
Gandhi et al. (2010) [39] 2 1 2 2 2 2 10
Gandhi et al. (2013) [21] 2 2 2 2 2 2 12
Hanusch et al. (2013) [27] 2 2 2 2 0 2 10
Jones et al. (2001) [41] 2 1 2 2 2 2 11
Jones et al. (2003) [10] 2 0 2 2 2 2 10
Judge et al. (2012) [25] 2 0 2 2 2 2 10
Kauppila et al. (2011) [4] 2 2 2 2 0 2 10
Lingard et al. (2004) [26] 2 0 2 2 2 2 10
Lingard et al. (2007) [38] 2 0 2 2 0 2 8
Lopez-Olivo et al. (2011) [18] 2 1 2 2 2 2 11
Neuburger et al. (2013) [23] 1 0 2 2 0 2 7
Papakostidou et al. (2012) [28] 2 1 2 2 2 2 11
Perruccio et al. (2012) [19] 2 2 2 2 2 2 12
Rajgopal et al. (2008) [20] 2 1 2 2 2 2 11
Ramaesh et al. (2013) [29] 1 2 2 2 0 2 9
Riddle et al. (2010) [22] 2 2 2 2 2 2 12
Smith et al. (2004) [52] 1 0 2 2 0 2 7
Sullivan et al. (2011) [30] 2 0 2 2 2 2 10
Wylde et al. (2012) [31] 1 2 2 2 0 2 9
Yakobov et al. (2014) [36] 1 0 2 2 2 2 9
TOTAL (mean ± SD) /12 1.71 ± 0.46 1.00 ± 0.84 2.00 ± 0.0 2.00 ± 0.0 1.00 ± 1.02 2.00 ± 0.0 9.69 ± 1.47
TOTAL (mean ± SD) /100 85.4 ± 23.1 50.0 ± 42.1 100 ± 0.0 100 ± 0.0 50.0 ± 50.8 100 ± 0.0 80.7  ± 12.2

Study participation: The study sample represents the population of interest with regard to key characteristics, sufficient to limit potential bias to the results
Study attrition: Loss to follow-up is unrelated to key characteristics (that is, the study data adequately represent the sample), sufficient to limit potential bias
Prognostic factor measurement: The prognostic factor of interest is adequately measured in study participants, sufficient to limit potential bias
Outcome measurement: The outcome of interest is adequately measured in study participants, sufficient to limit bias Confounding measurement and account: Important potential confounders are appropriately accounted for, limiting potential bias with respect to the prognostic factor of interest
Analysis: The statistical analysis is appropriate for the design of the study, limiting potential for the presentation of invalid results
SD: standard deviation