Table 3. Quality assessment of studies according to Newcastle-Ottawa Scale.

Study	Year	Selection				Comparability		Exposure			Total
		S1	S2	S3	S4	C1	C2	E1	E2	E3
Mamata	2005	*	*	*	*	*	*	*	*	*	9
Facon	2005	*	*	*	*	-	-	*	*	*	7
Xiangshui	2010	*	*	*	*	*	-	*	*	*	8
Kim	2010	*	*	*	*	-	-	*	*	*	7
Kang	2011	*	*	*	*	*	-	*	*	*	8
Lee	2011	*	*	*	*	*	*	*	*	*	9
Budzik	2011	*	*	*	*	*	*	*	*	*	9
Song	2011	*	*	*	*	*	*	*	*	*	9
Uda	2013	*	*	*	*	*	-	*	*	*	8
Wen	2013	*	*	*	*	*	-	*	*	*	8
Wen	2014	*	*	*	*	*	-	*	*	*	8
Banaszek	2014	*	*	*	*	*	-	*	*	*	8
Cui	2014	*	*	*	*	*	-	*	*	*	8
Rajasekaran	2014	*	*	*	*	-	-	*	*	*	7

S1: Selection1-is the case definition adequate; S2: Selection2-representativeness of the cases; S3: Selection3-selection of controls; S4: Selection4-definition of controls. C1: Comparability1-comparability of controls for most important factor; C2: Comparability2-comparability of controls for other factors. E1: Exposure1-ascertainment of exposure; E2: Exposure2-same method of ascertainment for cases and controls; E3: Exposure3-non-response rate.