1. Aberdeen Pain Research Collaboration (Epidemiology Group); Institute of Applied Health Sciences; University of Aberdeen, School of Medicine and Dentistry; Polwarth Building, Foresterhill; Aberdeen; AB25 2ZD; UK
2. MRC Centre of Epidemiology for Child Health/Centre for Paediatric Epidemiology and Biostatistics; UCL Institute of Child Health; London; WC1N 1EH; UK