The review addressed a clear question and some criteria for inclusion were defined. Appropriate databases were searched with no language restrictions. Attempts were made to identify studies from conference proceedings. Although the authors examined the results for publication bias using recognised methods, the small number of studies with small sample sizes means that the results of this analysis may not be valid. Steps were taken by the reviewers to minimise errors and bias for the selection of studies in the review, but were not reported for the quality assessment or the extraction of data.
Although the methodological quality of the studies were evaluated, the results of this assessment were not summarised in the review, so the reliability of the results was unknown and the statistical combination of the results in a meta-analysis may not have been appropriate. The results of observational studies were associated with a number of potential biases, so the results should be interpreted with substantial caution.
Some flaws in the review process and the lack of information about the quality of the observational studies included in the review mean the authors' conclusions should be interpreted with a substantial degree of caution and may not be reliable.