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Two hundred and forty-six subjects aged 60 to 94 were tested with the WAIS at test session 1, the start of a longitudinal study which lasted approximately 20 years. The subject attrition from the first session to the last (session No. 11) was progressive and selective, leaving mainly the intellectually superior ones in the study. Two facts appeared particularly important: One, little intellectual decline was seen in these superior subjects until very late in life. Two, analyses based on such superior subjects without reference to the larger body of subject populations can lead to spurious generalizations about aging in general. Age decline in intellectual ability seen in more representative populations may not be apparent when analyzing data of select samples.
Siegler et al. (Thu,) studied this question.