Data extraction
Three reviewers (LOB, SA, AOS) independently extracted data using a
standardised collection form for all eligible studies, including study
characteristics and reported outcomes for analysis. Where multiple
publications using the same data existed, we included the most recent
study, provided the earlier publication did not contain reported
information not available in the most recent study. In studies in which
adequate data was not reported, an effort was made to contact authors to
provide us with additional information to allow us to compute effect
estimates. Risk of bias assessment was undertaken by three reviewers
(LOB, SA, AOS) using an appropriate quality assessment tool described by
McDonald et al15, with all included studies classified
as low to moderate risk of bias.