Table 2.

Newcastle–Ottawa Scale quality assessment of each included study.

Study	Selection	Comparability	Exposure/Outcome	Total Score
Bulatao-Jayme et al., 1982 [35]	***	*	**	6
Parkin et al., 1991 [36]	***	**	*	6
Chao et al., 1994 [37]	*	-	-	1
Wang et al., 1996 [38]	***	**	*	6
Omer et al., 1998 [39]	***	**	*	6
Zang et al., 1998 [40]	***	**	*	6
Pillay et al., 2002 [41]	*	*	-	2
Wu et al., 2008 [42]	****	**	*	7
Wu et al., 2008 [43]	****	**	*	7
Persson et al., 2012 [44]a	****	**	*	7
Persson et al., 2012 [44]b	****	**	*	7
Chen et al., 2013 [49]	-	*	-	1
Lai et al., 2014 [45]	****	**	*	7
Belhassen et al., 2015 [46]	**	*	***	6
Ouhibi et al., 2020 [47]	***	*	*	5
Niknejad et al., 2021 [48]	**	*	*	4

Stars (*, **, ***, ****) stand for one, two, three or four scores in the assessment of study quality using modified NOS.