Table 2 Methodological quality of studies included in the meta-analysis.

	Selection (score)				Comparability (score)	Exposure (score)
Study	Adequate definition of patient case	Representativeness of patients cases	Selection of controls	Definition of control	Control for important factor or additional factor	Ascertainment of exposure (blinding)	Same method of ascertainment for participants	Non-response Rate^a	Total Score^b
Peng[15]	1	1	1	1	0	0	1	0	5
Peng[15]	1	1	1	1	0	0	1	0	5
Piao [16]	1	1	1	1	2	0	1	1	8
Gu [17]	1	1	0	1	2	0	1	0	6
Wei [18]	1	1	1	1	2	0	1	0	7
Dura [19]	1	1	1	1	2	0	1	0	7
Zhou[20]	1	1	1	1	2	0	1	0	7
Ren[21]	1	1	1	1	2	0	1	0	7
Shang [22]	1	1	1	1	1	0	1	0	6
Zhang [23]	1	1	1	1	2	0	1	1	8
Wang [10]	1	1	1	1	2	0	1	0	7
Wang[10]	1	1	1	1	2	0	1	0	7

^aWhen there was no statistical significance in the response rate between case and control groups by using a chi-squared test (P > 0.05), one point was awarded;
^bTotal score was calculated by adding up the points awarded in each item.

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