Search Results Use the search box below to revise your search, or start a new search. Studies (3) Variables Publications Your query returned 3 studies. Series results are grouped under the most relevant study. Ungroup series results to view all results. Sort by: Title A-Z Released/Updated Time Period Most Cited in ICPSR Bib. 1. Center for Research on Social Reality [Spain] Survey, May 1995: Old Age (ICPSR 6969) Centro de Investigaciones Sobre la Realidad Social (CIRES) 2. Danish 1905 Cohort Study, 1998 (ICPSR 3960) Christensen, Kaare; Duke University 3. Longitudinal Study of Aging Danish Twins, 1995 (ICPSR 21041) Christensen, Kaare; Vaupel, James W.