Why Should We Study Change in Cognitive Function?

There is little we prize as highly as our ability to think clearly, and little we fear as much as losing this ability. Unfortunately, increasing numbers of older persons will face this loss. The prevalence of Alzheimer’s disease, the most common dementia, increases strongly with age. In the United States and other developed countries, the oldest age groups of the population are also the most rapidly growing in numbers. Thus, we confront a public health problem that is already large and will continue to grow at an increasing pace. Meeting this challenge will require thoughtful progress in epidemiologic research as well as in other areas. Studying change in cognitive function, as Colsher and Wallace (Ann Epidemiol 199!;2:215-230) have done, is important to this progress. A major reason is avoidance of bias. It is difficult to measure cognitive function in an unbiased fashion. This is especially so in studies of such risk factors as education and other measures of socioeconomic status. Years of formal education and performance on cognitive tests are strongly correlated cross-sectional data (l-5). This association is difficult to interpret, however, because assessment of cognitive function among individuals with fewer years of formal education is likely biased. The use of change in cognitive function as an outcome appears to have substantial advantages in avoiding or reducing this bias. To the extent that any factors distorting measurement of cognitive function at the time of first measurement are similar to those affecting subsequent measurements, the difference between measurements will be unbiased. In addition, measures of change in cognitive function will be of practical importance to the development of studies of incident Alzheimer’s disease that are relatively free from bias. Probably, only a fraction of the cases of this disease are recognized in the routine delivery of medical care. Further, selective factors for recognition are unknown and may well be related to risk factors of interest. Studies of incident disease in defined populations that detect cases independently of the medical care system reduce this bias. The most practical case detection methods in such populations employ sequential administration of screening tests of cognitive function to detect decline in cognition, followed by a second stage of clinical evaluation of persons sampled according to the results of these tests. Such studies can have the additional value of providing more accurate rates of disease incidence and permitting description of the course of disease. Fortunately, use of sequential data has been an area of recent strong advances in statistical methodology. The attitude that analytic use of change was more likely to be deceptive than informative (6) has been largely replaced by a recognition of the value of direct measurement of change in studying processes in aging (7) and in other fields (8). The study reported by Colsher and Wallace combines two features that will be of increasing importance to advances in this field: consideration of change in cognitive function and a defined population. Future investigations will likely include other features including direct consideration of change, multiple measurement points, and

0 1991 Elsevier Science Publishing

Co., Inc.




clinical evaluation to provide information cognitive impairment.

AEP Vol. 1, No. 3 February 1991: 283-284

about specific conditions



Denis A. Evans, MD Center for Research


Health and Aging Rush-Presbyterian-St.


Medical Center Chicago,


REFERENCES 1. Birren JE, Morrison DF. Analysis of the WAIS subsets in relation to age and education, J Gerontol. 1961;16:363-369. 2. Pfeiffer E. A short portable mental status questionnaire for the assessment of organic brain deficit in elderly patients, J Am Geriatr Sot. 1975;23:433-41. 3. Gurland B, Copeland J, Kuriansky J, Kelleher M, Sharpe L, Dean LL. The Mind and Mood of Aging: Mental Health Problems of the Community Elderly in New York and London. New York: Haworth Press; 1983. 4. Scherr PA, Albert MS, Funkenstein HH, et al. Correlates of cognitive function in an elderly community population, Am J Epidemiol. 1988;128:1084-101. 5. Salmon DP, Riekkinen PJ, Katzman R, Zhang M, Jin H, Yu E. Cross-cultural studies of dementia; a comparison of mini-mental state examination performance in Finland and China, Arch Neurol. 1989;46:769-72. 6. Cronbach LJ, Furby L. How should we measure “change’‘-or should we?, Psycho1 Bull. 1970;1:68-80. 7. Rogosa D. Myths about longitudinal research. In: Schaie KW, Campbell RT, Meredith W, Rawlings SC, eds. Methodological Issues in Aging Research. New York: Springer; 1988:171-209. 8. Fletcher C, Peto R, Tinker C, Speizer FE. The Natural History of Chronic Bronchitis and Emphysema. London: Oxford University Press; 1976.

Why should we study change in cognitive function?

Why Should We Study Change in Cognitive Function? There is little we prize as highly as our ability to think clearly, and little we fear as much as l...
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