1 James White, Centre for Trials Research & Centre for the Development and Evaluation of Complex Interventions for Public Health Improvement (DECIPHer) Cardiff University, Cardiff CF14 4YS, UK, email: whitej11@cf.ac.uk; Paola Zaninotto, Department of Epidemiology and Public Health, University College London, London, UK; Mika Kivimäki, Department of Epidemiology and Public Health, University College London, London, and Institute of Behavioural Sciences, University of Helsinki, Helsinki, Finland; G. David Batty, Department of Epidemiology and Public Health, University College London, London, and Centre for Cognitive Ageing and Cognitive Epidemiology, University of Edinburgh, Edinburgh, UK.