Table 2. Results of quality assessment using the Newcastle-Ottawa Scale for case-control studies.

Study (year)	Selection					Comparability of cases and controls on the basis of the design or analysis		Exposure				Scores
	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	Non-response rate
Wu 2017	☆	☆		☆		☆☆		☆	☆	☆		8
Song 2018	☆	☆		☆		☆☆		☆	☆	☆		8
Cui 2019	☆	☆		☆		☆☆		☆	☆	☆		8
Yang 2018	☆	☆		☆		☆☆		☆	☆	☆		8
Chen 2018	☆	☆		☆		☆☆		☆	☆	☆		8