Table 2.

Risk of bias assessment of included studies.

Author (year)	Adequate sequence generation	Allocation concealment	Blinding of participants and personal	Blinding of outcome assessment	Incomplete outcome data	Selective reporting	Other bias
Blumenthal et al. (2005)12	✓	✓	✓	✓	✓	✓	✓
Huang et al. (2014)13	✓	✓	✓	✓	✓	✓	?
Maria Sarullo et al. (2006)15	✓	✓	✓	✓	✓	✓	?
Servantes et al. (2011)14	✓	✓	✓	✓	✓	✓	✓
Edelmann et al. (2011)16	✓	✓	✓	✓	✓	✓	✓
Chrysohoou et al. (2014)17	✓	✓	✓	✓	✓	✓	?
Westhoff-Bleck et al. (2013)18	✓	✓	?	✓	✓	✓	?
Ricca-Mallada et al. (2012)19	✓	✓	?	✓	✓	✓	✕
Fu et al. (2011)20	✓	✓	✓	✓	✓	✓	✓
Nishi et al. (2011)21	✓	✓	✕	?	✓	✓	?
Murad et al. (2012)22	✓	✓	?	?	✓	✓	✓
Osbak et al. (2011)23	✓	✕	✕	✓	✓	✓	?
Yeh et al. (2011)24	✓	✓	✓	✓	✓	✓	✕
Kitzman et al. (2010)26	✓	✓	✓	✓	✓	✓	✓
Pullen et al. (2009)27	✓	✓	✓	✓	✓	✓	✓
Brubaker et al. (2009)28	✓	✓	✓	✓	✓	✓	✓
Collins et al. (2003)29	✓	✓	✓	✓	✓	✓	?
Andersen et al. (2006)31	✓	✓	✓	✓	✓	✓	?
Selig et al. (2004)32	✓	✓	?	✓	?	✓	?
Conraads et al. (2007)33	✓	✓	?	✓	✓	✓	✕
Pozehl et al. (2010)34	✓	✓	✓	✕	✓	✓	✕
Jurgens et al. (2013)35	✓	✓	✓	✓	✓	✓	✓
Maiorana et al. (2011)25	✓	✓	✓	✓	✓	✓	✓
Belardinelli et al. (2008)30	✓	✓	✓	✓	✓	✓	✓

✕ = high risk; ✓ = low risk; ? = unclear risk.