Table 4.

Risk of bias assessment for the studies in the systematic review and meta-analysis.

	Feng et al.	Tran et al.	Armstrong et al.
Were the two groups similar and recruited from the same population?	Yes	Yes	Yes
Were the exposures measured similarly to assign people to both exposed and unexposed groups?	Yes	Yes	Yes
Was the exposure measured in a valid and reliable way?	Yes	Yes	Yes
Were confounding factors identified?	Yes	Yes	No
Were strategies to deal with confounding factors stated?	Yes	No	NA
Were the participants free of the outcome at the start of the study (or at the moment of exposure)?	Unclear	Unclear	Yes
Were the outcomes measured in a valid and reliable way?	Yes	Yes	Yes
Was the follow up time reported and sufficient to be long enough for outcomes to occur?	Yes	Yes	Yes
Was follow up complete, and if not, were the reasons to loss to follow up described and explored?	Yes	Yes	Yes
Were strategies to address incomplete follow up utilized?	NA	NA	NA
Was appropriate statistical analysis used?	Yes	Yes	Yes
Overall assessment	Low	Low	Low

NA: not applicable