Table 2. The quality assessment of included studies based on the Cochrane handbook.

Study	A	B	C	D	E	F	G	Total
Massie (1999)	?	?	+	?	+	+	+	4
Neill (2003)	?	?	+	+	+	+	+	4
Mador (2005)	+	?	-	?	+	+	+	4
Salgado (2008)	?	?	-	-	+	+	+	3
Ryan (2009)	?	?	+	+	+	+	+	4
Worsnop (2010)	?	?	+	+	+	-	+	3
Ruhle(2011)	?	?	?	?	+	+	+	3
Soudorn(2016)	?	?	+	?	+	+	+	4
Nilius(2016)	?	?	+	?	+	+	+	4

A = sequence generation; B = allocation concealment; C = blinding of participants and personnel; D = blinding of outcome assessment; E = incomplete outcome data; F = selective outcome reporting; G = other sources of bias; + = Yes;— = No; ? = unclear (‘Yes’ for a low risk of bias, ‘No’ for a high risk of bias; ‘Unclear’ otherwise)