Table 2.

Risk of bias of included studies according to the Newcastle–Ottawa Scale.

Author (year)	Selection	Comparability	Outcome/Exposure	Overall quality report
Gudmundsdottir et al. (2017) (17)	3	2	2	7
Silva et al. (2017a) (6)	2	2	2	6
Silva et al. (2017b) (28)	2	2	2	6
Van den Broek et al. (2017) (7)	2	2	2	6
Gudmundsdottir et al. (2016) (9)	3	2	2	7
Van den Broek et al. (2016) (10)	2	2	2	6
Zlamy et al. (2016) (29)	2	2	2	6
Elder et al. (2015) (18)	2	2	2	6
Schadenberg et al. (2014) (25)	3	2	2	7
Kurobe et al. (2013) (19)	2	1	2	5
Sauce et al. (2012) (27)	2	2	2	6
Cao et al. (2011) (20)	2	2	2	6
Van Gent et al. (2011) (16)	2	2	2	6
Eysteinsdottir et al. (2009) (30)	2	2	2	6
Prelog et al. (2009) (23)	2	2	2	6
Sauce et al. (2009) (24)	2	2	2	6
Mancebo et al. (2008) (21)	2	2	2	6
Torfadottir et al. (2006) (15)	2	2	2	6
Halnon et al. (2005) (26)	2	2	2	6
Eysteinsdottir et al. (2004) (31)	2	2	2	6
Wells et al. (1998) (22)	2	2	2	6
Ramos et al. (1996) (32)	2	2	2	6
Brearley et al. (1987) (3)	2	2	2	6