CRD commentary
The objective of the review was clear and the inclusion criteria were appropriate. The literature search was thorough
with attempts to identify unpublished literature. However, by limiting the review to English-language studies, relevant
studies could have been missed and the findings might be influenced by language bias. Since the validity of the
included studies was not assessed, it is unclear whether the results of the studies and the synthesis of them are reliable.
A narrative synthesis was appropriate given the differences between the studies. The authors did not describe how the
data were extracted, so it is not possible to determine whether steps were taken to minimise bias in this process. The
authors' conclusions should be interpreted in light of these limitations.