Table 2.

Risk of bias assessment of included studies.

Study	Selection				Comparability	Outcome			Score
	1	2	3	4	5	6	7	8
Biswas 2008[16]	*	*	*	*	**	*	*	*	9
Ji 2011[23]	*	*	*	*	**	*	*	*	9
Das 2012[27]	*	*	*	*	**	*	*	*	9
Karim 2013[28]	*	*	*	*	**	*	*		8
Ning 2014[25]	*	*	*	*	*		*	*	7
Dutta 2015[29]	*	*	*	*	**		*	*	8
Wang 2016[26]	*	*	*	*	*		*	*	7
Prasad 2017[17]	*	*	*	*	*	*	*	*	8
Gao 2018[30]	*	*	*	*	**	*	*	*	9
Xu 2020[31]	*	*	*	*	**	*	*	*	9
Fang 2019[24]	*	*	*	*	**		*	*	8
Tutkun 2019[18]	*	*	*	*	*		*	*	7
Rahman 2021[19]	*	*	*	*	**	*	*	*	9

(1) Sufficient definition of the cases (2) Representativeness of the cases (3) Selection of the controls (4) Definition of the controls (5) Comparability of cases and controls based on the design or analysis (6) Assessment of exposure (7) Same method of ascertainment for cases and controls (8) non-response rate.