The review question and inclusion criteria were clear. Efforts were made to find published studies but unpublished studies were not sought. No evidence of publication bias was found, but funnel plots for less than 10 studies are not very meaningful, so publication bias could not be fully ruled out. No language restrictions were applied, which reduced potential language bias. Attempts were made to minimise reviewer errors and bias in the review process.
The authors reported that the study quality was assessed but full results were not reported, so the quality of the studies were unknown. Appropriate methods were used to pool data and assess heterogeneity.
Although the authors' conclusions reflected the available evidence, the reliability of their conclusions is limited due to the lack of reported quality assessment, small sample sizes, and inclusion of mainly non-randomised studies.