Table 2.

Evaluation of risk of bias.

Study	Q1	Q2	Q3	Q4	Q5	Q6	Q7	Q8	Q9	Q10	TOTAL
Akolekar et al., 2009 [22]	YES	YES	YES	YES	YES	YES	UNC	UNC	YES	YES	8
Aoba et al., 1967 [23]	NO	NO	UNC	NO	NO	NO	NO	NO	YES	NO	1
Bagga et al., 1969 [24]	YES	NO	NO	NO	NO	NO	NO	NO	UNC	UNC	1
Bolin et al., 2009 [25]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Chen et al., 2021 [26]	UNC	YES	YES	YES	YES	YES	YES	YES	YES	YES	9
Gotsch et al., 2008 [27]	YES	YES	YES	YES	YES	UNC	YES	YES	YES	YES	9
Han et al., 2012 [28]	YES	YES	YES	YES	YES	UNC	YES	YES	NO	YES	8
Hirokoshi et al., 2005 [29]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Hirokoshi et al., 2007 [30]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Horjus et al., 2019 [31]	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	10
Kamal et al., 2011 [32]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Karakus et al., 2015 [33]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Khalil et al., 2014 [34]	YES	YES	YES	YES	YES	YES	YES	YES	UNC	YES	9
Koroglu et al., 2018 [35]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Kumar et al., 2011 [36]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Leinonen et al., 2009 [38]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Leijnse et al., 2018 [37]	YES	YES	YES	YES	YES	YES	NO	YES	YES	YES	9
Machado et al., 2019 [39]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Martínez et al., 2018 [40]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Mazibuko et al., 2019 [41]	YES	UNC	YES	YES	YES	NO	NO	YES	YES	UNC	6
Morrison et al., 1971 [42]	NO	NO	UNC	YES	UNC	NO	NO	UNC	YES	NO	2
Nadar et al., 2005 [43]	YES	YES	YES	YES	YES	UNC	UNC	YES	YES	YES	8
Naghshvar et al., 2013 [44]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Nayel et al., 1982 [45]	YES	UNC	UNC	NO	UNC	NO	NO	NO	YES	NO	2
Puttapitakpong et al., 2015 [46]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Aref et al., 2013 [47]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Salgo et al., 1989 [48]	NO	NO	NO	NO	UNC	NO	NO	NO	UNC	UNC	0
Sammour et al., 1974 [49]	YES	YES	UNC	YES	YES	NO	NO	YES	UNC	YES	6
Sammour et al., 1975 [50]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Schneuer et al., 2013 [51]	YES	NO	YES	UNC	UNC	NO	NO	YES	YES	YES	5
Shim et al., 2015 [52]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Sung et al., 2011 [54]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Wang et al., 2011 [55]	YES	YES	YES	YES	YES	NO	NO	YES	UNC	YES	7
Watson et al., 1965 [56]	NO	NO	YES	UNC	UNC	NO	NO	UNC	YES	UNC	2
Young et al., 1968 [57]	NO	YES	YES	YES	YES	NO	NO	YES	YES	YES	7
Kim et al., 2013 [53]	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	10
Zeng et al., 2009 [58]	YES	YES	YES	YES	YES	NO	NO	YES	YES	YES	8
Q1. Were the groups comparable other than the presence of disease in cases or the absence of disease in controls? Q2. Were cases and controls matched appropriately? Q3. Were the same criteria used for the identification of cases and controls? Q4. Was exposure measured in a standard, valid, and reliable way? Q5. Was exposure measured in the same way for cases and controls? Q6. Were confounding factors identified? Q7. Were strategies to deal with confounding factors stated? Q8. Were outcomes assessed in a standard, valid, and reliable way for cases and controls? Q9. Was the exposure period of interest long enough to be meaningful? Q10. Was appropriate statistical analysis used?

Green—Low risk of bias. Yellow—Unclear risk of bias. Red—High risk of bias.