Table 2.

Results of quality assessment using the Newcastle–Ottawa scale for case-control studies.

First author, year	Selection				Comparability of cases and controls on the basis of the design or analysis	Exposure
	Adequate definition of cases	Representativeness of the cases	Selection of controls	Definition of controls		Ascertainment of exposure	Same method of ascertainment for cases and controls	Nonresponse rate	Scores
Li, 2018 [18]	☆	☆		☆	☆☆	☆	☆	☆	8
Cheng et al., 2018 [19]	☆	☆		☆	☆☆	☆	☆	☆	8
Shao et al., 2019 [20]	☆	☆		☆	☆☆	☆	☆	☆	8
Liu et al., 2020 [21]	☆	☆		☆	☆☆	☆	☆	☆	8
Zhang and Shi, 2020 [22]	☆	☆		☆	☆☆	☆	☆	☆	8
Zhang and Xiong, 2019 [23]	☆	☆		☆	☆☆	☆	☆	☆	8
Zhao, 2018 [24]	☆	☆		☆	☆☆	☆	☆	☆	8

☆: study can be awarded a maximum of one star for each numbered item within the selection and exposure categories; ☆☆: a maximum of two stars can be given for comparability.