Identifying and separating the effects of practice and of cognitive ageingduring a large longitudinal study of elderly community residents

In protracted longitudinal studies of cognitive changes in old age voluntee rs must be repeatedly tested. Even with intervals of several years between assessment, this raises the possibility that improvements due to practice m ask other changes. This problem is much more acute in brief studies of cogn itive changes associated with progressive pathologies such as Alzheimer's d isease or the effects of clinical interventions. Both types of study also e ncounter problems of selective dropout of frail and less able individuals l eaving relatively 'elite' survivors. An analysis of data from repeated test ing at 2-3 years intervals on the AH4 (1) intelligence test is presented to illustrate how a random effects model can be used to identify and disassoc iate age-related changes and practice effects at the population level, afte r effects of selective dropout and of background demographical variables ha ve been taken into consideration. This analysis also provides some new, sub stantive empirical findings. Age-related changes are relatively slight betw een 49 and 70 years but much more marked between 70 and 80 years. Even with assessment points, several years apart the population average effect of pr actice is large relative to that of age-related change. Variation between i ndividuals increases as samples age, providing the first clear evidence fro m a longitudinal study for marked individual differences in trajectories of cognitive ageing. (C) 2001 Elsevier Science Ltd. All rights reserved.