In cross-sectional comparisons of their original scores, nonsurviving, older subjects and those who refused to be retested scored consistently below retestees. These results point to the heterogeneity of the aging population and to biases in the sampling process. Analysis of scores by going backward in age (negative age) starting with the time of death as the zero point suggests the occurrence of lower limits in performance (lethal limit). Decline with age is attributed to a sudden drop in performance occurring within 5 years prior to the death of subjects (terminal drop). Throughout adulthood, performances of long-term survivors are unchanged. The decline with age observed in cross-sectional studies is attributed to the increasing number of subjects exhibiting terminal drops.
Intelligence, verbal abilities, attitudes, interests, and social conditions of 380 Ss above 55 yr. of age were measured. Five yr. later Ss were retested. Some refused to cooperate again, and others had died or become ill. Retested Ss differed significantly from the total group, but in particular from the other subgroups. The prediction of death on the basis of sociopsychological variables was more successful for Ss below than above 65 yr. of age. It was concluded that developmental trends are based on increasingly biased samples, that previous studies have underestimated the amount of attrition, and that nonsurvivors under 65 yr. form a sociopsychological subgroup of different characteristics than survivors.
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