Publication bias
There was no evidence of publication bias in the pooled estimates for risk factors involving three or more studies based on the results of Egger’s test (p>0.05) as shown in Table 3. Publication bias could not be assessed in pooled data involving only two studies. In the sensitivity analysis, no significant changes were evident in the observed directions and effect sizes following the random removal of a single study in each turn. We could not explore the funnel plot symmetry because of the small number of studies.