Table 2.

Assessment of risk of bias in individual studies

Study	Selection bias	Performance bias	Detection bias	Attrition bias	Reporting bias
Busch et al. (2013)	H	H	H	H	L
de Leeuw et al. (2010)	H	H	H	H	L
Korkmaz and Kiran-Esen (2012)	U	H	H	H	L
Turel et al. (2015)	H	H	H	H	L
Walther et al. (2014)	L	H	H	L	L

Note. H: high risk of bias; L: low risk of bias; U: unclear bias. We applied strict criteria in the assessment. However, it must be noted that, for example, performance and detection biases are, in general, rather uncommon in these types of studies. Perhaps, in these studies, U could also be used for performance and detection biases.