Table 2.

Quality appraisal by Cochrane Collaboration’s tool for assessing risk of bias

No.	Domain	Selection bias	Performance bias	Detection bias	Attrition bias	Reporting bias	Other bias
	Reference
1	Kim,2014	+	?	+	+	+	?
2	Stanosz,2009	+	+	+	+	+	?
3	Ettinger,2004	+	+	+	+	+	?
4	Davas,2003	+	?	+	+	?	?
5	Pereda,2002	+	+	+	+	+	?
6	Yang,2007	+	?	+	+	+	?
7	Adami ,1989	+	+	?	+	+	?
8	Alexandersen,1999	+	+	+	+	+	?
9	Gonnelli,1997	+	+	+	+	+	?

Key: Low risk: +; Unclear risk:? ; High risk: -