Table 2.

Quality of bias assessment of the included studies according to the Cochrane guidelines

Study	Sequence generation	Allocation concealment	Blinding of participants, personnel and outcome assessors	Incomplete outcome data	Selective outcome reporting	Other sources of bias
Balan et al. (1994) [18]	U	U	L	L	L	U
Battezzati et al. (1993) [21]	L	L	L	L	L	L
Braga et al. (2009) [22]	U	U	U	L	L	U
Carulli et al. (1981) [32]	U	U	U	L	L	U
Fromm et al. (1983) [23]	U	U	U	L	L	U
Gianturco et al. (2013) [39]	L	L	U	L	L	U
Leuschner et al. (2010) [40]	U	U	U	L	L	U
Lindenthal et al. (2002) [33]	U	U	H	L	L	U
Méndez-Sánchez et al. (2004) [19]	L	L	U	L	L	U
Miettinen et al. (1995) [20]	U	U	U	L	L	U
Nakagawa et al. (1977) [37]	U	L	L	L	L	U
Parés et al. (2000) [34]	U	L	L	L	L	U
Poupon et al. (1990) [35]	U	U	U	L	L	U
Poupon et al. (1993) [38]	H	U	U	L	L	U
Vuoristo et al. (1995) [36]	U	U	U	L	L	U

L low risk of bias, H high risk of bias, U unclear risk of bias