Table 3. Quality assessment of the included studies according to the Newcastle–Ottawa scale.

Studies	Selection (4 stars in total, 1 for each item)				Comparability (2 stars)	Outcome (3 stars in total, 1 for each item)			Total score
	Representativeness of the exposed cohort	Selection of the nonexposed cohort	Assessment of exposure	Outcome not present at start of study		Assessment of outcome	Follow-up long enough for outcomes	Adequacy of follow-up
Sun [16]	1	1	1	1	2	1	0	0	7
Zhang [17]	1	1	1	1	2	0	0	0	6
Qin [18]	1	1	1	1	2	1	0	0	7
Xu [22]	1	1	1	1	1	1	1	0	7
Li [23]	1	1	1	1	1	1	1	0	7
Liu [24]	1	1	1	1	1	1	0	0	6
Schalper [25]	1	1	1	1	2	1	0	0	7
Wang [26]	1	1	1	1	1	1	1	1	8
Xu [27]	1	1	1	1	2	1	0	0	7