Competing interests: We have read and understood BMJ policy on conflicts of interest and declare the following: MC has received personal fees from Astellas, Takeda, and Glaukos outside the submitted work and grants from the NIHR and is a coinvestigator at Health Data Research UK Midlands. DK is supported by the NIHR postdoctoral fellowship scheme (PDF-2016-09-009). JMV has received grants and fellowships from NIHR and Instituto de Salud Carlos III (Spain), grants from MRC, CRUK, Royal Devon and Exeter Trust, Fondo de Investigaciones Sanitarias (Spain), consultancy fees from the World Health Organization, and Technische Krankenkasse (Germany) and currently is chairman (unpaid) of the International Consortium for Health Outcomes Measurement panel for the development of core sets of outcomes for Health Overall Adult Health. GP and NHH declare no competing interests.