Footnotes

• Competing interests None.

• Funding The Centre for Paediatric Epidemiology and Biostatistics was supported in part by the Medical Research Council in its capacity as the MRC Centre of Epidemiology for Child Health. Research at the University College London Institute of Child Health and Great Ormond Street Hospital for Children receives a proportion of funding from the Department of Health's National Institute for Health Research Biomedical Research Centres funding scheme.

• Provenance and peer review Commissioned; externally peer reviewed.