The Danish 1905 Cohort

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Abstract
Objectives: The authors studied nonagenarians, a rapidly growing age group whose cognitive and physical abilities have yet to be investigated systematically. Methods: All Danes born in 1905 were invited to participate in a home-based 2-hour multidimensional interview, including cognitive and physical performance tests and collection of DNA, carried out by lay interviewers. Population-based registers were used to evaluate representativeness. Results: There were 2,262 participants. A total of 1,632 (72%) gave a DNA sample. Participants and nonparticipants were highly comparable with regard to marital status, institutionalization, and hospitalization patterns, but men and rural area residents were more likely to participate. Six months after the survey began, 7.2% of the participants and 11.8% of the nonparticipants had died. Discussion: Despite the known difficulties of conducting surveys among the extremely old, it was possible to conduct a nationwide survey, including collection of DNA, among more than 2,000 fairly nonselected nonagenarians using lay interviewers.