Table 2.

Quality assessment of included studies using the Newcastle-Ottawa Scale

First author	Selectiona				Comparabilityb	Exposurec			Score
	1	2	3	4		A	B	C
Cen [16]	☆	☆	-	☆	☆	☆	☆	☆	7
Fedchuk [11]	☆	☆	-	-	☆	☆	☆	☆	6
Fu [14]	☆	☆	☆	☆	☆	☆	☆	☆	8
Keskinler [18]	☆	☆	-	☆	☆☆	☆	☆	☆	8
Li [12]	☆	☆	-	☆	☆☆	☆	☆	☆	8
Lin [17]	☆	☆	☆	☆	☆	☆	☆	☆	8
Musso [10]	☆	☆	-	-	☆	☆	☆	☆	6
Ortega [15]	☆	☆	-	-	☆	☆	☆	☆	6
Vassilatou [13]	☆	☆	-	☆	☆	☆	☆	☆	7

Rank by the beginning letter of the first authors

^a1: Is the case definition adequate? 2: Representativeness of the cases. 3: Selection of Controls. 4: Definition of Controls

^bComparability: Comparability of cases and controls on the basis of the design or analysis. A maximum of two stars can be given for Comparability

^cA: Ascertainment of exposure. B: Same method of ascertainment for cases and controls. C: Nonresponse rate