The review addressed a clear question and inclusion criteria were defined. However, the authors stated that an expert excluded studies based on external validity and quality; this is not a systematic approach and may have introduced bias into the review, although it was unclear how many (if any) studies were excluded on this basis. The literature search included some relevant sources, but use of a diagnostic filter, restriction to English-language studies and lack of an explicit search for unpublished studies meant that relevant studies may have been missed. Appropriate steps were taken to minimise bias and errors during the review process. Study quality was assessed using appropriate criteria and the results were clearly presented, but were not considered in the synthesis. Very few details on the participants in the included studies were reported, which made it difficult to determine the generalisability of the findings. Sophisticated methods were used to pool data; however, heterogeneity was not formally assessed or investigated.
Limitations in the review including the possibility of missing studies and failure to investigate heterogeneity together with the small number, size and variable quality of the included studies mean that the authors' conclusions are unlikely to be reliable.