Table 3.

Quality of included case-control studies and cross-sectional studies based on the modified version of Newcastle–Ottawa scale.

Study	Selection				Comparability	Outcome			Total Score
	Adequate case definition☆	Representativeness of cases☆	Selection of controls☆	Definition of controls☆	Comparability of cases and controls on basis of design or analysis☆☆	Exposure ascertainment☆	Same method of ascertainment for cases and controls☆	Non-response rate☆
Jung JH, 2018, Korea	☆	–	–	☆	☆☆	☆	☆	☆	7
Zhou M, 2018, China	☆	–	–	☆	☆☆	☆	☆	–	6
Von Mühlen D, 2002, USA	☆	☆	☆	☆	☆☆	☆	☆	☆	9
Sandmark H, 1999, Sweden	☆	☆	☆	☆	☆☆	☆	☆	–	8
Sowers MF, 1996, USA	☆	–	☆	☆	☆☆	☆	☆	☆	8