The Centre for Diet and Activity Research (CEDAR) [1] is an interdisciplinary Centre of Excellent in Public Health Research, a partnership between the University of Cambridge, the University of East Anglia and MRC Units in Cambridge. It studies influences and interventions on diet and physical-activity behaviours, and helps to shape public-health policy and practice [2]. Among its current research programmes is ‘Behavioural epidemiology of young people’s activity behaviour’, which is looking at the factors that influence young people’s physical activity, and developing and evaluating population-level interventions [3].