Selection of studies
The titles and abstracts of all studies were screened independently by two authors (AT/MDW) against the inclusion and exclusion criteria. Where there was uncertainty regarding eligibility, or there was disagreement between authors, the full texts of articles were obtained and reviewed. There were no differences of opinion that were not resolved through discussion. Where title and abstracts were identified in English language, but the main report was in a foreign language, they were subsequently excluded from analysis.