About the Authors
Institute of Child Health, University of Liverpool, Liverpool, United Kingdom
Centre for Medical Statistics and Health Evaluation, University of Liverpool, Liverpool, United Kingdom
The authors have declared that no competing interests exist.
IS, RLS, and PRW designed the study protocol and search strategy. IS and LJ identified the relevant studies from the search results. IS and LJ extracted data, which were checked by PRW and RLS. IS, LJ, RLS, and PRW were involved in data analysis. IS prepared the initial manuscript. IS, RLS, and PRW were all substantially involved in the revision of this manuscript. All authors checked the final manuscript before submission.