Table 4.

The risk of bias of the included studies

Author (year)	Selection bias		Performance bias		Detection bias	Attrition bias	Reporting bias	Other bias
	Random sequence generation	Allocation concealment	Blinding of personnel	Blinding of participants	Blinding of outcome assessment	Incomplete outcome data	Selective reporting
Benzo et al. (2016)	+	?	+	−	+	+	+	+
Bischoff et al. (2012)	+	?	+	−	?	+	+	+
Cameron‐Tucker et al. (2016)	+	+	+	−	+	+	+	+
Greening et al. (2014)	+	+	+	−	+	+	+	+
Johnson‐Warrington et al. (2016)	+	+	?	−	+	+	+	+
Jolly et al. (2018)	+	+	+	−	?	+	+	+
Khdour et al. (2009)	+	?	−	−	−	+	+	+
Mitchell et al. (2014)	+	+	?	−	+	+	+	+
Song et al. (2014)	?	?	?	−	?	+	+	+
Walters et al. (2013)	+	+	−	−	?	+	+	+

+ low risk of bias, − high risk of bias, ? unclear risk of bias.