Cohort profile: Oxford Pain, Activity and Lifestyle (OPAL) Study, a prospective cohort study of older adults in England oleh Christian D Mallen, M Watson, S Dutton, S Petrou, Lesley Ward, Maria T Sanchez Santos, Angela Garrett, Alana Morris, O Conway, F Darton, D Hagan, D Haywood, A Hewitt, I Marian, L Nevay, P Nicolson, M Slark, K Vadher, E Williamson, N Arden, K Barker, G Collins, J Fairbank, J Fitch, D French, F Griffiths, Z Hanson, C Hutchinson