1. Institute of Biomedical and Clinical Science; Exeter Medical School; University of Exeter; Exeter; EX2 5DW; UK
2. Epidemiology and Public Health; Exeter Medical School; University of Exeter; Exeter; EX2 5DW; UK
3. Institute of Health Service Research; Exeter Medical School; University of Exeter; Exeter; EX2 4SG; UK
4. Laboratory of Neurogenetics; National Institute on Aging; National Institutes of Health; Bethesda; MD; USA
5. Geriatric Unit; Azienda Sanitaria di Firenze; Florence; Italy
6. Laboratory of Epidemiology, Demography and Biometry; National Institute on Aging; Bethesda; MD; USA
7. National Institute on Aging; Baltimore; MD; USA