Fig. 2

Risk of bias assessment using STROBE statement for each study. The statement consisted of a checklist of 22 items. Each item was classified as “no” (high risk), “yes” (low risk), or “unclear” to facilitate assessing the risk of potential bias in the title and abstract, introduction, methods, results, discussion, and other information sections of articles. In order to report the results in percentages, the total number of “no”, “yes”, or “unclear” was added and divided by the total number of items for each study and multiplied by 100