Table 3.

NOS risk of bias evaluation.

Reference	Selection	Comparability	Exposition	Conclusion
[17]	★★★★	★	★★	Low risk
[36]	★★★★	★	★★★	Low risk
[43]	★★★★	★	★★★	Low risk
[41]	★★★★	★	★★★	Low risk
[44]	★★★★	★	★★★	Low risk
[42]	★★★★	★	★★★	Low risk
[45]	★★★★	★	★★★	Low risk
[37]	★★★★	★	★★★	Low risk
[39]	★★★★	★	★★★	Low risk
[40]	★★★★	★	★★★	Low risk
[38]	★★★★	★	★★★	Low risk
[46]	★★★★	★	★★★	Low risk

★ indicates the quality of the studies, when the sum of the ★ is <4: low-quality study; 4–6 ★: moderate-quality study and ≥7 ★: high-quality study.