Journals of Gerontology Series B: Psychological Sciences and Social Sciences
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The Journals of Gerontology Series B: Psychological Sciences and Social Sciences 59:P84-P97 (2004)
© 2004 The Gerontological Society of America

RESEARCH ARTICLE

Practice and Drop-Out Effects During a 17-Year Longitudinal Study of Cognitive Aging

Patrick Rabbitt1,, Peter Diggle2, Fiona Holland2 and Lynn McInnes3

1 Age and Cognitive Performance Research Centre, University of Manchester, England.
2 Department of Biomathematics and Statistics, University of Lancaster, England.
3 Department of Psychology, University of Northumbria, England.

Address correspondence to Patrick Rabbitt, Age and Cognitive Performance Research Centre, Zochonis Building, University of Manchester, Manchester, M13 9PL, United Kingdom. E-mail: rabbitt{at}psy.man.ac.uk

Interpretations of longitudinal studies of cognitive aging are misleading unless effects of practice and selective drop-out are considered. A random effects model taking practice and drop-out into account analyzed data from four successive presentations of each of two intelligence tests, two vocabulary tests, and two verbal memory tests during a 17-year longitudinal study of 5,899 community residents whose ages ranged from 49 to 92 years. On intelligence tests, substantial practice effects counteracted true declines observed over 3 to 5 years of aging and remained significant even with intervals of 7 years between successive assessments. Adjustment for practice and drop-out revealed accelerating declines in fluid intelligence and cumulative learning, linear declines in verbal free recall, and no substantial change in vocabulary. Socioeconomic status and basal levels of general fluid ability did not affect rates of decline. After further adjustment for demographics, variability between individuals was seen to increase as the sample aged.






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