Table 2.

Assessment of risk of bias in individual studies

Study	Selection bias		Reporting bias	Performance bias	Detection bias	Attrition bias	Other bias
	Sequence generation	Allocation concealment					Sampling bias	Measurement bias
Stadler et al. (2007)	–	?	–	?	?	–	+	+
Zepf et al. (2008a)	–	?	–	?	?	–	+	+
Zepf et al. (2008c)	–	?	–	?	?	?	+	+
Zepf et al. (2008b)	–	?	–	?	?	?	+	+
Zepf et al. (2009b)	–	?	–	?	?	?	+	+
Zepf et al. (2009a)	–	?	–	?	?	?	+	+
Zepf et al. (2010)	–	?	–	?	?	?	+	+
Zimmermann et al. (2012)	–	?	–	?	?	?	+	+
Mette et al. (2013)	–	?	–	?	?	?	+	+
Zepf et al. (2013)	–	?	–	?	?	?	+	+
Kötting et al. (2013)	–	?	–	?	?	?	+	+
Grabemann et al. (2013)	–	?	–	?	?	?	+	+
Von Polier et al. (2014)	–	?	–	?	?	–	+	+
Biskup et al. (2016)	–	?	–	?	?	?	+	–

– low risk

+ high risk

^? unclear risk