The objectives and inclusion criteria of the review were clear. Relevant sources were searched for studies, without restriction by language. It was unclear whether there was any restriction by publication status, but the authors found no evidence of publication bias when this was assessed. It was unclear whether steps were taken to minimise the risk of bias by having more than one reviewer independently select studies and extract the data.
It did not appear that study validity was assessed. Study methods were not described (e.g. selection procedures and rates of follow-up). These factors made it difficult to determine the reliability of the evidence presented. Appropriate statistical techniques were used to combine the data and assess statistical heterogeneity.
In view of the lack of prospective evidence, poor reporting of review methods and failure to assess study validity, some caution in interpreting the authors’ conclusions may be advisable.