D A V I D J. E K E R D T
Book Review Essay A N A R C H I V E OF F I N D I N G S ON A G I N G
Erdman Palmore, Ewald W. Busse, George L. Maddox, John B. Nowlin and Ilene C. Siegler, eds. 1985. Normal Aging III: Reports from the Duke Longitudinal Studies, 1975--1984. Durham, N. C., Duke University Press. 468 pp., $32.50. This is the third collection of papers under the Normal Aging title from the Duke University Longitudinal Studies, the previous volumes having incorporated reports from 1955 to 1969 (Palmore 1970) and from 1970 to 1973 (Palmore 1974). The present book invites some comment about the institutional parent of this research, about the papers as a group, and about the characterization of change and aging.
Longitudinal studies of aging and human development are known for their major goals, measures, and lines of research; for their conclusions; and for the teams of investigators who gather and analyze their data. Primarily, however, a longitudinal study is identified with its population, because the population is the study. In order to describe changes within individuals, a panel of human participants is enrolled and measured repeatedly over a period of years. Although research on aging can profitably proceed by other designs, the description of within-individual change across adulthood requires the observation of people as they are aging. Longitudinal studies thus become known for the populations they keep. The Duke longitudinal studies encompassed two panels of human participants. The first was developed for the Duke Longitudinal Study of Aging beginning in 1955, a date not far removed from the beginning of organized gerontology in the United States. In 1955, the field had yet to hear about a disengagement theory, or ponder the age-period-cohort conundrum, or take for granted computer-assisted data management and analysis. This first Duke study was to be descriptive and exploratory, with "normal aging" conceived to mean typical developmental processes as distinct from pathology. The study format would be the repeated observation of medical and psychological functioning. Toward this end, a group of 270 community-dwelling men and women aged 60 to 90 were recruited from the older adult population in the region around Durham, North Carolina. The participant burden for this study would be heavy, entailing two-day examinations at the Duke Medical Center that would consist of Journal of Cross-Cultural Gerontology 3 (1988), 87--94. © 1988 by KluwerAcademic Publishers.
88
D A V I D J. E K E R D T
protocols to gather biomedical, behavioral, and social variables. Under these circumstances any randomly selected population would soon experience attrition and eventually become a self-selected group. So from the outset the investigators wisely relied on a population of socially heterogeneous volunteers in order to maximize the continued participation of the panelists. This population would be reexamined 11 times at irregular intervals over the next 20 years until 1976 when data collection was closed off, with the final round of examinations conducted on 44 individuals. A second panel was formed for the Adaptation Study that commenced in 1968. The goal of this program, more specific than the first, was to study responses to special problems of adaptation during middle age and the pre-retirement years, but attention was also given to aging processes generally and the characteristics of successful aging. This time the investigators sampled white members of a large Durham health insurance association in order to fill 10 age and gender cohorts. The sampling strategy for this panel (half the people contacted agreed to join) yielded a noninstitutionalized, middle and upper-income population of 502 men and women aged 46--70. Panelists underwent four rounds of testing -- an 8-hour multidisciplinary battery -- every two years. Data collection was completed in 1976 on 375 surviving members. Neither the first nor second panel was paid for its participation, but many persons found benefit in the free medical examinations and the satisfaction of contributing to a useful research program. Normal Aging III comes some ten years after the cessation of routine data collection in both longitudinal studies. Its publication accompanies that of another, slimmer volume, The Duke Longitudinal Studies of Normal Aging, 1955--1980 (Busse and Maddox 1985). Of the two, the former is a repository of findings, the latter a self-described overview of the Duke effort. By coincidence, another major American study, the ongoing Baltimore Longitudinal Study of Aging, weighed in at the same time with a retrospective report of its first 23 years (Shock, Greulich, Andres, Arenberg, Costa, Lakatta and Tobin 1984). This last book combines the features (findings and context) that are separated in the two Duke volumes. Normal Aging III includes 47 different contributions by 38 different authors or co-authors. These are sorted into four sections, each introduced by one of the volume's editors: Physical Aging introduced by John Nowlin, Mental Health and Mental Illness by Ewald Busse, Psychological Aging by Ilene Siegler, and Social Aging by George Maddox. Senior editor Erdman Palmore has written a brief Preface and Summary. Twenty-nine papers in the volume were previously published in other sources, seven were read at scientific meetings, and eleven had not appeared elsewhere. The collection is a bit more dated than the years (1975--1984) indicated
BOOK REVIEW ESSAY
89
by the book's subtitle. The editors reached back for papers published or read in 1973. Only one previously published paper appeared after 1982. The unpublished papers may represent more recent work, though none of these references any other source published after 1982. Finally, it is not said whether this collection includes all of the empirical work generated from the Duke data in this period, or only selected material. Given the extensive examinations administered to the Duke participants, a great array of measures in biomedical and psychosocial domains were available for analysis, and this collection reflects that variety. There are papers on sexuality, longevity, mental performance, cerebral blood flow, sleep patterns, stress, memory, personality, self-concept, health perceptions, retirement, religious activity -- and this is a partial list. This diversity of topics is something that the four section introductions cannot quite overcome and knit together. While these papers do not cohere as a set, that was not the point of this publication, which was rather to bind together reports that were based on a common resource. Looming over this collection is a reader's curiosity about the institutional parent of this output. Alas, the natural history of these papers is not a subject of this book. The two-page preface is very brief about the goals and design of the two studies -- this material having been covered in Normal Aging and Normal Aging H and elsewhere -- and the book is practically mute about other details of this ambitious, long-running, and productive program. It makes a fascinating story nonetheless: what was this organization that attracted a community of investigators and motivated it, how was scientific continuity maintained over the years, what were the procedures for managing this formidable data set, who was entitled to analyze data or institute new routines, and -- most significantly -- who paid for all of this? Readers interested in these questions can consult the Busse and Maddox (1985) publication for a partial account. II
Of what use is such a collection of papers? The specialist in such topics as memory, longevity, or stress should have encountered relevant papers in their originally published sources or come to note them from the programs of scientific meetings. The reader with a general interest in aging might sample these reports, but should realize that each belongs to a wider literature on its respective topic, and its conclusions should be weighed in the context of that evolving body of knowledge. Has further research on this topic by other investigators taken note of this paper, used its methods, replicated its findings, or confirmed its conclusions? Single papers may be informative, but they are rarely definitive, especially when -- as Normal Aging authors regularly note -- findings are based on selected samples from one part of the world and a certain historical period in time. As
90
D A V I D J. E K E R D T
knowledge about aging cumulates from gerontological research, some papers in this collection will be judged as greater contributions to the literature than others, but their significance is a matter to be evaluated by specialist readers. Specific contributions aside, this volume will be important as a resource for historians of gerontology and research on human development. It is also a convenient book of record that readers can consult to satisfy themselves about the exact nature of findings which are only summarized in other review articles, book chapters, and monographs about the Duke studies. When it comes to evaluating longitudinal studies, it is well to keep in mind that it is usually easier to be critical than generous toward these programs. No matter how luminous in some aspects, longitudinal studies always get mixed reviews. They are such sprawling affairs that they cannot be impressive in all their particulars. Mostly they are vulnerable to hindsight about their methods -- early design decisions about the nature of the population, the choice of measures, and the frequency of data collection. (Hindsight is usually not so hard on theoretical rationales, which are assumed to be perishable.) Certain central protocols or lines of inquiry may be rendered obsolescent by other advances in the discipline. Not all domains, in the case of a multidisciplinary study, may have been equally well developed or carefully justified. The panel may later be judged to be too homogeneous, to have been a little too young or too old at baseline, or to have been re-measured at inappropriate intervals. Such complaints are more visible now that outside investigators can access the data tapes from several longitudinal studies (including the Duke studies) through centralized data archives. When results of these secondary analyses are presented at scientific meetings, one too often hears statements of regret that the original study was not conducted more to the investigator's liking. This, then, is the essential trade-off: the longitudinal study may not, after a time, be using state-of-the-art methods, but then state-of-the-art methods will never, by definition, be used longitudinally. Given that the durability of the data set is an asset, another appropriate question about this collection is how well these papers exploit the longitudinality of the data. Most of these reports were written at a time when the data collection in both studies had been completed and multiple waves of data were available for analysis. Many of the Normal Aging III authors took advantage of this. By one count, slightly more than half of the papers were specifically concerned with within-individual change in some variable, characteristic, or status, and the majority of these prospective reports involved data from three or more occasions of measurement. Several other papers used multiwave data not to track within-individual change but to report, for example, the prevalence of mental illness, or to study the early predictors of a later outcome, like longevity. About a
BOOK REVIEW ESSAY
91
quarter of the reports in this book could be classified as cross-sectional analyses, usually of data from one-shot ancillary studies. However, even a few cross-sectional reports were nevertheless informed by the longitudinal record in their use of population or methods. The years after 1976, with the full matrix of data now available, also presented an opportunity for Duke investigators to write the major empirical papers from the longitudinal studies on the central issues of aging and adaptation that the research was originally designed to address. The reader of Normal Aging III can discern that some of these papers are in that class; for example, Gianturco and Busse's review of psychiatric problems in later life, the paper by McCarty et al. on patterns of memory scores, or the analysis by Palmore et al. of stress and adaptation. The editors, however, might have been a little more forthcoming in suggesting which papers came closest to fulfilling the primary objectives of either study. III
The Normal Aging III collection will inform some readers about specific issues in aging and prompt others to reflect on the accomplishments of these particular longitudinal studies. The book also prompts some reflection about the business of studying change and how change is used to characterize aging. Two such issues, one conceptual and the other methodological, concern the nature of "continuity" as a finding, and the problem of attrition bias in longitudinal data analysis. Operationally, there is no such thing as aging, there is only change -change in structure, function, and behavior. Longitudinal studies measure change in order to make inferences about the nature of aging. Not all change is aging, and not all age-related change is normal aging. The investigators from the Baltimore Longitudinal Study (Shock et al. 1984: 208) have usefully listed six types or patterns of change relevant to aging. One is stability, or the absence of meaningful change. Another is gradual decline with age that is illness-related. A third pattern is decline in function regardless of health (this comes closest to the idea of normal or normative aging). A fourth pattern is precipitous change, often prompted by disease. The fifth type is apparent change that is compensatory, representing the body's or person's attempt to maintain function with advancing age. The sixth kind of change over time is due to cultural changes that alter behavior. A good number of papers in Normal Aging III report conclusions about change of the first kind -- which is no change. The "Summary and Future" section notes the continuity or stability of an array of variables and lists "little or no decline in social and psychological functioning" as one of the five "themes" of the collection (other themes are: "advantages of
92
DAVID J. EKERDT
longitudinal and interdisciplinary study of aging", "patterns of declining health and physical functioning", "exceptions to physical decline", and "wide variation in aging patterns"). For example, Duke investigators found stability in measures of general intelligence, personality, age identification, sexual activity, life satisfaction, religious attitudes, and more. Stability meant not only the maintenance of mean levels of a variable but also (in papers by Maddox and Douglass and by Fox) described individuals' unchanging rank relative to peers on multiple psychosocial indicators. "Continuity" findings deserve some caution. One set of authors pointed out that aggregated statistics "can mask or blur distinct patterns of change and stability exhibited by individuals" (George and Weiler, p. 12). Another author (Fox, p. 391) warned that "measurement is the message", i.e., error in unreliable measures can overwhelm the observation of "true" change, or else measures may be too broad to capture the meaningful dynamic aspects of a concept. To these should be added the idea that individuals may not have been followed long enough to have exhibited significant change in some variables. Continuity of psychological and social functioning, given that it is a valid finding, is a theme not only of the Duke studies but of findings from other longitudinal studies as well. However, as a conclusion it is unsatisfactory, not because it is inaccurate, but rather because it wants more explanation. Continuity is an ambiguous, residual characterization of the "no change" pattern -- a sort of positive thing to say about null findings when the goal was to describe some feature of aging processes. It is possible to interpret continuity as evidence that aging is not all decline and decay. There is cheer in this, especially when it refutes common myths of aging. What is really wanted, however, is an a c c o u n t of continuity, equilibrium, consistency, or resilience across time. What makes people appear to have an evenness of attitudes and activities, to conserve their self-concept and orientation, to maintain their intellectual abilities, or to defend their outlook even in the face of potentially disruptive life events? Suspicion, of course, falls to personality as the substrate of continuity, but there may also be a role for social forces in pulling individuals toward stable, consistent behavior (see George in this volume, pp. 249--250). Research on mechanisms of compensation and equilibrium is part of a new agenda for gerontology and one that will find itself using designs other than the periodic measurement of aging panels. Nonetheless, a debt is due to programs like the Duke longitudinal studies for disclosing the extent of continuity in human functioning as well as individual variability in patterns of aging. One cannot think how else we would know these things. Another significant issue that concerns change and how it is characterized is the matter of attrition from the panel. Though administrators of longitudinal studies go to great lengths to conserve their populations, some panelists will drop out due to death, incapacity, or loss of interest before
BOOK REVIEW ESSAY
93
the data collection ends. What is more, nonparticipation in the study will tend to be nonrandom and systematically related to central variables of interest, like health. The remaining group of long-term survivors in the panel, the "complete" cases on whom full data is available from all rounds, are likely to be an elite group who enjoy advantaged life circumstances and who function better than their peers in the original panel as well as the general population. This was confirmed by one of Normal Aging III's more instructive papers, in which Siegler and Botwinick analyzed "Selective Attrition and Intellectual Ability" in the first Duke panel. They found that less able people tended not to survive to the later rounds of the study. The lesson was that "the more stringent the testing demands are on the subject, the more it is mainly the intellectually, and perhaps physically, superior who live up to those demands" (p. 200). Thus, attrition can produce bias in the characterization of change, skewing observations toward overestimates of stable functioning or underestimates of decline. Approximately two-fifths of the papers in Normal Aging III examined data from three or more waves of either panel study. In most instances, the strategy for dealing with dropouts was to exclude them from the analysis. Future analysts of these data and data from other longitudinal studies, if they are to adequately describe age-related change, must face more squarely this issue of survivorship bias. If a panel ages by ten or twenty years, the experience of individuals who survived only the first five years is a necessary part of that panel's experience of aging. However, except for the analysis of events or transitions, the statistical models that can handle partial observations, truncated records, and missing rounds of data are not readily at hand. Yet statistical approaches to the problem of incomplete longitudinal series will be developed, and once they are implemented, the existing Duke data sets can be used anew to establish more precise estimates about change and stability in later life. The founders of the Duke studies knew that their contribution to gerontology would be in raising issues as well as settling them. The legacy of these longitudinal studies should also be appreciated in terms of the extensive training activities that surrounded them and the careers that they launched; in terms of the highly visible leadership that Duke investigators have provided within gerontology and their respective disciplines; and in terms of the valuable data sets that now reside in the public domain available for further analyses. When the chronicle of gerontology is written, the Duke longitudinal studies will be seen to have had a profound period and generational effect on the study of human aging. REFERENCES
CITED
Busse, E. W. and Maddox, G. L. 1985 The Duke Longitudinal Studies of Normal Aging, 1955 -- 1980. New York: Springer.
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Palmore, E., ed. 1970 Normal Aging. Durham, N. C.: Duke University Press. Palmore, E., ed. 1974 Normal Aging II. Durham, N. C.: Duke University Press. Shock, N., Greulich, R. C., Andres, R., Arenberg, D., Costa, P. T., Lakatta, E. G. and Tobin, J. D. 1984 Normal Human Aging: The Baltimore Longitudinal Study of Aging. NIH Publication No. 84-2450. Washington, D.C.: U.S. Government Printing Office.
Veterans Administration Normative Aging Study Boston, Mass.
Book Review Essay A N A R C H I V E OF F I N D I N G S ON A G I N G
Erdman Palmore, Ewald W. Busse, George L. Maddox, John B. Nowlin and Ilene C. Siegler, eds. 1985. Normal Aging III: Reports from the Duke Longitudinal Studies, 1975--1984. Durham, N. C., Duke University Press. 468 pp., $32.50. This is the third collection of papers under the Normal Aging title from the Duke University Longitudinal Studies, the previous volumes having incorporated reports from 1955 to 1969 (Palmore 1970) and from 1970 to 1973 (Palmore 1974). The present book invites some comment about the institutional parent of this research, about the papers as a group, and about the characterization of change and aging.
Longitudinal studies of aging and human development are known for their major goals, measures, and lines of research; for their conclusions; and for the teams of investigators who gather and analyze their data. Primarily, however, a longitudinal study is identified with its population, because the population is the study. In order to describe changes within individuals, a panel of human participants is enrolled and measured repeatedly over a period of years. Although research on aging can profitably proceed by other designs, the description of within-individual change across adulthood requires the observation of people as they are aging. Longitudinal studies thus become known for the populations they keep. The Duke longitudinal studies encompassed two panels of human participants. The first was developed for the Duke Longitudinal Study of Aging beginning in 1955, a date not far removed from the beginning of organized gerontology in the United States. In 1955, the field had yet to hear about a disengagement theory, or ponder the age-period-cohort conundrum, or take for granted computer-assisted data management and analysis. This first Duke study was to be descriptive and exploratory, with "normal aging" conceived to mean typical developmental processes as distinct from pathology. The study format would be the repeated observation of medical and psychological functioning. Toward this end, a group of 270 community-dwelling men and women aged 60 to 90 were recruited from the older adult population in the region around Durham, North Carolina. The participant burden for this study would be heavy, entailing two-day examinations at the Duke Medical Center that would consist of Journal of Cross-Cultural Gerontology 3 (1988), 87--94. © 1988 by KluwerAcademic Publishers.
88
D A V I D J. E K E R D T
protocols to gather biomedical, behavioral, and social variables. Under these circumstances any randomly selected population would soon experience attrition and eventually become a self-selected group. So from the outset the investigators wisely relied on a population of socially heterogeneous volunteers in order to maximize the continued participation of the panelists. This population would be reexamined 11 times at irregular intervals over the next 20 years until 1976 when data collection was closed off, with the final round of examinations conducted on 44 individuals. A second panel was formed for the Adaptation Study that commenced in 1968. The goal of this program, more specific than the first, was to study responses to special problems of adaptation during middle age and the pre-retirement years, but attention was also given to aging processes generally and the characteristics of successful aging. This time the investigators sampled white members of a large Durham health insurance association in order to fill 10 age and gender cohorts. The sampling strategy for this panel (half the people contacted agreed to join) yielded a noninstitutionalized, middle and upper-income population of 502 men and women aged 46--70. Panelists underwent four rounds of testing -- an 8-hour multidisciplinary battery -- every two years. Data collection was completed in 1976 on 375 surviving members. Neither the first nor second panel was paid for its participation, but many persons found benefit in the free medical examinations and the satisfaction of contributing to a useful research program. Normal Aging III comes some ten years after the cessation of routine data collection in both longitudinal studies. Its publication accompanies that of another, slimmer volume, The Duke Longitudinal Studies of Normal Aging, 1955--1980 (Busse and Maddox 1985). Of the two, the former is a repository of findings, the latter a self-described overview of the Duke effort. By coincidence, another major American study, the ongoing Baltimore Longitudinal Study of Aging, weighed in at the same time with a retrospective report of its first 23 years (Shock, Greulich, Andres, Arenberg, Costa, Lakatta and Tobin 1984). This last book combines the features (findings and context) that are separated in the two Duke volumes. Normal Aging III includes 47 different contributions by 38 different authors or co-authors. These are sorted into four sections, each introduced by one of the volume's editors: Physical Aging introduced by John Nowlin, Mental Health and Mental Illness by Ewald Busse, Psychological Aging by Ilene Siegler, and Social Aging by George Maddox. Senior editor Erdman Palmore has written a brief Preface and Summary. Twenty-nine papers in the volume were previously published in other sources, seven were read at scientific meetings, and eleven had not appeared elsewhere. The collection is a bit more dated than the years (1975--1984) indicated
BOOK REVIEW ESSAY
89
by the book's subtitle. The editors reached back for papers published or read in 1973. Only one previously published paper appeared after 1982. The unpublished papers may represent more recent work, though none of these references any other source published after 1982. Finally, it is not said whether this collection includes all of the empirical work generated from the Duke data in this period, or only selected material. Given the extensive examinations administered to the Duke participants, a great array of measures in biomedical and psychosocial domains were available for analysis, and this collection reflects that variety. There are papers on sexuality, longevity, mental performance, cerebral blood flow, sleep patterns, stress, memory, personality, self-concept, health perceptions, retirement, religious activity -- and this is a partial list. This diversity of topics is something that the four section introductions cannot quite overcome and knit together. While these papers do not cohere as a set, that was not the point of this publication, which was rather to bind together reports that were based on a common resource. Looming over this collection is a reader's curiosity about the institutional parent of this output. Alas, the natural history of these papers is not a subject of this book. The two-page preface is very brief about the goals and design of the two studies -- this material having been covered in Normal Aging and Normal Aging H and elsewhere -- and the book is practically mute about other details of this ambitious, long-running, and productive program. It makes a fascinating story nonetheless: what was this organization that attracted a community of investigators and motivated it, how was scientific continuity maintained over the years, what were the procedures for managing this formidable data set, who was entitled to analyze data or institute new routines, and -- most significantly -- who paid for all of this? Readers interested in these questions can consult the Busse and Maddox (1985) publication for a partial account. II
Of what use is such a collection of papers? The specialist in such topics as memory, longevity, or stress should have encountered relevant papers in their originally published sources or come to note them from the programs of scientific meetings. The reader with a general interest in aging might sample these reports, but should realize that each belongs to a wider literature on its respective topic, and its conclusions should be weighed in the context of that evolving body of knowledge. Has further research on this topic by other investigators taken note of this paper, used its methods, replicated its findings, or confirmed its conclusions? Single papers may be informative, but they are rarely definitive, especially when -- as Normal Aging authors regularly note -- findings are based on selected samples from one part of the world and a certain historical period in time. As
90
D A V I D J. E K E R D T
knowledge about aging cumulates from gerontological research, some papers in this collection will be judged as greater contributions to the literature than others, but their significance is a matter to be evaluated by specialist readers. Specific contributions aside, this volume will be important as a resource for historians of gerontology and research on human development. It is also a convenient book of record that readers can consult to satisfy themselves about the exact nature of findings which are only summarized in other review articles, book chapters, and monographs about the Duke studies. When it comes to evaluating longitudinal studies, it is well to keep in mind that it is usually easier to be critical than generous toward these programs. No matter how luminous in some aspects, longitudinal studies always get mixed reviews. They are such sprawling affairs that they cannot be impressive in all their particulars. Mostly they are vulnerable to hindsight about their methods -- early design decisions about the nature of the population, the choice of measures, and the frequency of data collection. (Hindsight is usually not so hard on theoretical rationales, which are assumed to be perishable.) Certain central protocols or lines of inquiry may be rendered obsolescent by other advances in the discipline. Not all domains, in the case of a multidisciplinary study, may have been equally well developed or carefully justified. The panel may later be judged to be too homogeneous, to have been a little too young or too old at baseline, or to have been re-measured at inappropriate intervals. Such complaints are more visible now that outside investigators can access the data tapes from several longitudinal studies (including the Duke studies) through centralized data archives. When results of these secondary analyses are presented at scientific meetings, one too often hears statements of regret that the original study was not conducted more to the investigator's liking. This, then, is the essential trade-off: the longitudinal study may not, after a time, be using state-of-the-art methods, but then state-of-the-art methods will never, by definition, be used longitudinally. Given that the durability of the data set is an asset, another appropriate question about this collection is how well these papers exploit the longitudinality of the data. Most of these reports were written at a time when the data collection in both studies had been completed and multiple waves of data were available for analysis. Many of the Normal Aging III authors took advantage of this. By one count, slightly more than half of the papers were specifically concerned with within-individual change in some variable, characteristic, or status, and the majority of these prospective reports involved data from three or more occasions of measurement. Several other papers used multiwave data not to track within-individual change but to report, for example, the prevalence of mental illness, or to study the early predictors of a later outcome, like longevity. About a
BOOK REVIEW ESSAY
91
quarter of the reports in this book could be classified as cross-sectional analyses, usually of data from one-shot ancillary studies. However, even a few cross-sectional reports were nevertheless informed by the longitudinal record in their use of population or methods. The years after 1976, with the full matrix of data now available, also presented an opportunity for Duke investigators to write the major empirical papers from the longitudinal studies on the central issues of aging and adaptation that the research was originally designed to address. The reader of Normal Aging III can discern that some of these papers are in that class; for example, Gianturco and Busse's review of psychiatric problems in later life, the paper by McCarty et al. on patterns of memory scores, or the analysis by Palmore et al. of stress and adaptation. The editors, however, might have been a little more forthcoming in suggesting which papers came closest to fulfilling the primary objectives of either study. III
The Normal Aging III collection will inform some readers about specific issues in aging and prompt others to reflect on the accomplishments of these particular longitudinal studies. The book also prompts some reflection about the business of studying change and how change is used to characterize aging. Two such issues, one conceptual and the other methodological, concern the nature of "continuity" as a finding, and the problem of attrition bias in longitudinal data analysis. Operationally, there is no such thing as aging, there is only change -change in structure, function, and behavior. Longitudinal studies measure change in order to make inferences about the nature of aging. Not all change is aging, and not all age-related change is normal aging. The investigators from the Baltimore Longitudinal Study (Shock et al. 1984: 208) have usefully listed six types or patterns of change relevant to aging. One is stability, or the absence of meaningful change. Another is gradual decline with age that is illness-related. A third pattern is decline in function regardless of health (this comes closest to the idea of normal or normative aging). A fourth pattern is precipitous change, often prompted by disease. The fifth type is apparent change that is compensatory, representing the body's or person's attempt to maintain function with advancing age. The sixth kind of change over time is due to cultural changes that alter behavior. A good number of papers in Normal Aging III report conclusions about change of the first kind -- which is no change. The "Summary and Future" section notes the continuity or stability of an array of variables and lists "little or no decline in social and psychological functioning" as one of the five "themes" of the collection (other themes are: "advantages of
92
DAVID J. EKERDT
longitudinal and interdisciplinary study of aging", "patterns of declining health and physical functioning", "exceptions to physical decline", and "wide variation in aging patterns"). For example, Duke investigators found stability in measures of general intelligence, personality, age identification, sexual activity, life satisfaction, religious attitudes, and more. Stability meant not only the maintenance of mean levels of a variable but also (in papers by Maddox and Douglass and by Fox) described individuals' unchanging rank relative to peers on multiple psychosocial indicators. "Continuity" findings deserve some caution. One set of authors pointed out that aggregated statistics "can mask or blur distinct patterns of change and stability exhibited by individuals" (George and Weiler, p. 12). Another author (Fox, p. 391) warned that "measurement is the message", i.e., error in unreliable measures can overwhelm the observation of "true" change, or else measures may be too broad to capture the meaningful dynamic aspects of a concept. To these should be added the idea that individuals may not have been followed long enough to have exhibited significant change in some variables. Continuity of psychological and social functioning, given that it is a valid finding, is a theme not only of the Duke studies but of findings from other longitudinal studies as well. However, as a conclusion it is unsatisfactory, not because it is inaccurate, but rather because it wants more explanation. Continuity is an ambiguous, residual characterization of the "no change" pattern -- a sort of positive thing to say about null findings when the goal was to describe some feature of aging processes. It is possible to interpret continuity as evidence that aging is not all decline and decay. There is cheer in this, especially when it refutes common myths of aging. What is really wanted, however, is an a c c o u n t of continuity, equilibrium, consistency, or resilience across time. What makes people appear to have an evenness of attitudes and activities, to conserve their self-concept and orientation, to maintain their intellectual abilities, or to defend their outlook even in the face of potentially disruptive life events? Suspicion, of course, falls to personality as the substrate of continuity, but there may also be a role for social forces in pulling individuals toward stable, consistent behavior (see George in this volume, pp. 249--250). Research on mechanisms of compensation and equilibrium is part of a new agenda for gerontology and one that will find itself using designs other than the periodic measurement of aging panels. Nonetheless, a debt is due to programs like the Duke longitudinal studies for disclosing the extent of continuity in human functioning as well as individual variability in patterns of aging. One cannot think how else we would know these things. Another significant issue that concerns change and how it is characterized is the matter of attrition from the panel. Though administrators of longitudinal studies go to great lengths to conserve their populations, some panelists will drop out due to death, incapacity, or loss of interest before
BOOK REVIEW ESSAY
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the data collection ends. What is more, nonparticipation in the study will tend to be nonrandom and systematically related to central variables of interest, like health. The remaining group of long-term survivors in the panel, the "complete" cases on whom full data is available from all rounds, are likely to be an elite group who enjoy advantaged life circumstances and who function better than their peers in the original panel as well as the general population. This was confirmed by one of Normal Aging III's more instructive papers, in which Siegler and Botwinick analyzed "Selective Attrition and Intellectual Ability" in the first Duke panel. They found that less able people tended not to survive to the later rounds of the study. The lesson was that "the more stringent the testing demands are on the subject, the more it is mainly the intellectually, and perhaps physically, superior who live up to those demands" (p. 200). Thus, attrition can produce bias in the characterization of change, skewing observations toward overestimates of stable functioning or underestimates of decline. Approximately two-fifths of the papers in Normal Aging III examined data from three or more waves of either panel study. In most instances, the strategy for dealing with dropouts was to exclude them from the analysis. Future analysts of these data and data from other longitudinal studies, if they are to adequately describe age-related change, must face more squarely this issue of survivorship bias. If a panel ages by ten or twenty years, the experience of individuals who survived only the first five years is a necessary part of that panel's experience of aging. However, except for the analysis of events or transitions, the statistical models that can handle partial observations, truncated records, and missing rounds of data are not readily at hand. Yet statistical approaches to the problem of incomplete longitudinal series will be developed, and once they are implemented, the existing Duke data sets can be used anew to establish more precise estimates about change and stability in later life. The founders of the Duke studies knew that their contribution to gerontology would be in raising issues as well as settling them. The legacy of these longitudinal studies should also be appreciated in terms of the extensive training activities that surrounded them and the careers that they launched; in terms of the highly visible leadership that Duke investigators have provided within gerontology and their respective disciplines; and in terms of the valuable data sets that now reside in the public domain available for further analyses. When the chronicle of gerontology is written, the Duke longitudinal studies will be seen to have had a profound period and generational effect on the study of human aging. REFERENCES
CITED
Busse, E. W. and Maddox, G. L. 1985 The Duke Longitudinal Studies of Normal Aging, 1955 -- 1980. New York: Springer.
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D A V I D J. E K E R D T
Palmore, E., ed. 1970 Normal Aging. Durham, N. C.: Duke University Press. Palmore, E., ed. 1974 Normal Aging II. Durham, N. C.: Duke University Press. Shock, N., Greulich, R. C., Andres, R., Arenberg, D., Costa, P. T., Lakatta, E. G. and Tobin, J. D. 1984 Normal Human Aging: The Baltimore Longitudinal Study of Aging. NIH Publication No. 84-2450. Washington, D.C.: U.S. Government Printing Office.
Veterans Administration Normative Aging Study Boston, Mass.
