Table 2. Quality assessment of case-control studies based on the Newcastle-Ottawa Scale.

Case-control study (n=3)	Selection				Comparability	Expose			Total
	Adequate definition of cases	Representative cases	Selection of controls	Definition of controls	Comparability of cases and controls	Exposure ascertainment	Same ascertainment criteria for cases and controls	Non-response rate
Lee et al. (2015)62	☆	☆	☆	☆	☆	-	☆	☆	7
Suh et al. (2001)88	☆	☆	-	☆	☆☆	-	☆	-	6
Nam et al. (2003)65	☆	☆	-	☆	☆☆	-	☆	-	6