Table 1.

Quality assessment of included studies.

Studies	Definition of Cases	Representativeness of Cases	Selection of Controls	Definition of Controls	Comparability	Exposure	Total
[18]	+	+	-	-	+-	++-	6
[19]	+	+	+	+	+-	++-	7
[20]	+	+	-	-	+-	++-	6
[21]	+	+	+	-	+-	++-	6
[22]	+	+	+	+	+-	++-	7
[23]	+	+	+	+	+-	++-	7
[24]	+	+	-	-	+-	++-	5
[25]	+	+	-	-	+-	++-	5
[26]	+	+	-	-	+-	++-	6
[27]	+	+	-	-	+-	++-	5
[28]	+	+	+	-	+-	++-	6
[29]	+	+	-	-	+-	+++	6
[30]	+	+	+	+	+-	++-	7
[31]	+	+	+	+	+-	+--	6
[32]	+	+	-	-	+-	++-	5
[33]	+	+	+	-	+-	+++	7
[34]	+	+	-	-	+-	++-	5
[35]	+	+	+	-	+-	++-	6
[36]	+	+	+	+	+-	++-	7
[37]	+	+	+	-	+-	++-	6
[38]	+	+	+	-	+-	++-	6
[39]	+	+	+	-	+-	++-	6

+: star assigned; -: star not assigned.