1 Institute of Health Informatics, University College London, London, United Kingdom, Health Data Research UK London, University College London, London, United Kingdom.
2 Great Ormond Street Institute of Child Health, University College London, London, United Kingdom.
3 Medical Statistics Department, London School of Hygiene and Tropical Medicine, London, United Kingdom.
4 Institute of Epidemiology & Health Care, University College London, London, United Kingdom.