This review’s protocol is based on a peer-reviewed protocol registered
with the Joanna Briggs Institute6 and covers Englishlanguage
literature published between 1975 and 2010. The search
strategy, exclusion criteria, details of study selection, quality assessment,
and data extraction have been previously published.8 In brief,
a medical librarian conducted the literature search. Two independent
reviewers assessed abstracts and articles against eligibility criteria
and critically appraised the methodological quality using established
criteria (eAppendix in Supplement).