DORLYJ.H. DEEGAND GERARDH. VAN DER ZANDEN

EXPERIENCES FROM LONGITUDINAL STUDIES OF AGING: A N I N T E R N A T I O N A L P E R S P E C T I V E 1,2

ABSTRACT. A review of current longitudinal studies of aging was undertaken to obtain background information for the initiation of new studies. Through interviews with investigators information was obtained on the aspects of population and sampling, topic selection, data collection, data quality, and collaboration. All investigators emphasized the importance of broad, multidisciplinary data collection to meet future research interests. Differences between studies are mainly related to organization, sampling design, and monitoring of data quality. Key Words: aging, longitudinal studies, methodology, study design

INTRODUCTION In aging research, the longitudinal design is increasingly recognized as the main tool to study aging processes and intrinsic and extrinsic factors influencing these processes. It is also recognized that the longitudinal design involves organizational and methodological complexities that are not easily mastered. Mastery of these complexities is the major determining factor of scientific output. Despite a four-decade long tradition of aging studies using the longitudinal design, little of the experience from these studies has found its way to literature. Practical problems, solutions, and alternative choices often remain unnoticed. This contribution reviews some of these problems and their solutions as chosen in three major longitudinal studies. Discussed are determination of sample, selection of topics, data quality, and interdisciplinary collaboration. Since pooling of expertise is desirable both for standardization of measurements and for joining various disciplinary perspectives, aspects relevant for collaborative projects are highlighted. The discussion of these aspects is preceded by a brief review of longitudinal studies in an international context. Since the authors are most familiar with the Dutch situation, the starting point of this brief review will be the development of longitudinal studies in the Netherlands.

A REVIEW OF LONGITUDINAL STUDIES OF AGING Definition o f Longitudinal Research

The studies reviewed are generally multidisciplinary, having a focus on epidemiology, biology, or psychology. They are based on samples of the general population. They are concerned with "aging" in a broad sense and are based at least partially on a sample of persons aged 65 years and over. The definition of Journal of Cross-Cultural Gerontology 6: 7-22, 1991. © 1991 KluwerAcademic Publishers. Printed in the Netherlands.

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DORLYJ.H. DEEGAND GERARDH. VAN DER ZANDEN

longitudinal research adhered to in this article is: A study is considered to be longitudinal, when the same persons are observed with respect to the same characteristics more often than two times and during a period long enough to enable the ascertainment of changes in these characteristics (Deeg 1989). This definition excludes several study designs which are widely referred to as longitudinal. Most obviously excluded is the so-called "follow-up" study, which uses only a baseline assessment of the participants and follow-up information of some outcome measure or "endpoint" - for example, mortality. It is true that many good studies, the results of which have played an important role, were based on this design. However, change can be studied best using three or more times of measurement (Nesselroade, Stigler, and Baltes 1980; Rogosa 1988). In addition, problems typical of longitudinal studies become more pronounced as the number of times of measurement increases beyond two (Deeg 1989). The inventory of studies for the current review was aided by previous inventories (Deeg 1983; Mednick and Baert 1981; Migdal, Abeles, and Sherrod 1981; Shock, Greulich, Andres, Arenberg, Costa, Lakatta, and Tobin 1984:19-44), literature searches, personal communications, and serendipity. Dutch Longitudinal Studies o f Aging

In the Netherlands, so far only a few studies fulfill the above criteria. Their main characteristics are listed in Table I. The Dutch Longitudinal Study among the Elderly's (DLSE) original objective was to describe the health status and possibly related social and psychological characteristics in the Dutch elderly (Van Zonneveld 1961). It was initiated in 1955 and conducted by the Netherlands Organization for Applied Scientific Research (TNO). Only after completion of the first survey was it decided to conduct a series of follow-up examinations. Especially during the early followup examinations, the study lacked sufficient administrative support. The sixth follow-up examination was carried out in 1974-75. A final, seventh follow-up survey of the 26 survivors was carried out in 1983, when the study had found a new home in the Department of Public Health and Social Medicine at Erasmus University in Rotterdam. In this new phase, vital status and causes of death were updated and predictors of survival time were studied. Predictors included physical as well as mental and social characteristics (Deeg, Van Zonneveld, Van der Maas, and Habbema 1989). The survivors distinguished themselves from their late contemporaries by a generally good health status and an easy-going attitude toward growing older and life in general (Van Zonneveld, Deeg, Van Tol, and Van der Schaft-Kleywegt 1987). The Nijmegen Relocation Project was essentially a psychological study. It was initially conducted in the former Gerontology Center where the Departments of Social Gerontology and Social Medicine, University of Nijmegen, and the Joint Institute of Applied Psychology collaborated, and was continued in the

EXPERIENCES FROM LONGITUDINAL STUDIES TABLE I Dutch longitudinal studies of older adults: Main characteristics Study

Start

End

Nature of sample

N

(Source: Van Zonneveld 1961, Deeg et al. 1985) 3149 Dutch 1955 1983 National Longiprobability tudinal sample Study through among practitiothe ners Elderly

Sex and ages at entry

Test interval

Test period

Main variables

M&F 65-99

2-5 yr

2 1/2 hr

Clinical anthropometry biochemistry cognitive hlth.care use med.history fam.history well-being social part.n

(Source: Coleman 1976. Wimmers et al. 1987, Mertens 1988) Nijmegen 1974 1986 sample of 454 M&F Relocaelderly 56+ tion stratified Project by housing

3 hr 6 mo. (after 1979: one more follow-up)

(Source: Kromhout 1983, Kromhout and Obermann-de Boer 1986) Seven 1960 1990 4/9 845 M 1 yr Counsample + 40-59 through tries of small 490 1973 Study town in + Nether+ 1985 5 yr lands 2/3 of nonexaminees in 1985

4 hr

Well-being psychology social part.n activity health

Clinical biochemistry anthropometry fam. history nutrition life style demography (from 1985:) activity health psychology

Department of Social Gerontology, University of Nijmegen. It was designed to study adaptation after relocation to a residential h o m e for the elderly or to "intermediary", adapted housing. Samples of persons who were expected to relocate were compared to those expected to stay in the c o m m u n i t y and those already living in a home. D u r i n g the five years subsequent to 1974, an intensive interview schedule was m a i n t a i n e d ( C o l e m a n 1976). Relocation to a residential h o m e for the elderly appeared to improve the self-esteem o f w o m e n , b u t not o f m e n ( W i m m e r s , Buijssen, and Mertens 1987). After a lapse, due to insufficient staffing, vital status was ascertained, and the survivors were reinterviewed in 1986. F a v o r a b l e subjective health and an engaged attitude towards life were associated with l o n g e r survival time; frequency o f relocation did not affect survival time (Mertens 1988).

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DORLYJ.H. DEEGAND GERARDH. VAN DER ZANDEN

The Zutphen study was initiated in the context of the collaborative effort of the Seven Countries Study. According to an international standardized protocol, sixteen samples of men aged 40-59 years were examined around 1960 and followed over a period of 10 years in order to study nutritional and other precursors of cardiovascular disease (Kromhout 1983). The Dutch study was conducted by the Department of Social Medicine, University of Leyden. In the 1980s, the original cohort reached retirement age. The study was "revived" in 1985. The surviving sample was supplemented with a sample from the unexamined part of the original Zutphen population. In Finland, Italy, and Yugoslavia, the study was revived as well, in close collaboration. The focus shifted to more general aspects of aging, including nutrition, life style, functional and emotional aspects (Kromhout and Obermann-de Boer 1986). Life style correlates of total and HDL cholesterol appeared to be similar across countries in 1985 (Kromhout, Nissinen, Menotti, Bloemberg, Pekkanen, and Giampaoli 1990). The final data collection cycle is taking place in 1990. In addition to these three studies, several studies that are planned to be longitudinal have just finished their first data collection phase. The Department of Human Nutrition of the Agricultural University of Wageningen is conducting a study of nutrition in the elderly. This is a multi-center study according to the EURONUT protocol in which 16 European centers currently participate. The Section of Gerontology in the Faculty of Medicine, University of Leyden, is conducting a community study of persons aged 85 years and over with a special focus on the immune system. The Department of Epidemiology and Health Care Research of the University of Limburg is involved in a community study of persons aged 65 and over on functional disability in relation to health services use. A similar study is being conducted on a smaller scale by the Department of Health Sciences, University of Groningen. Several other longitudinal studies are being planned, with pilots ongoing. Two of these are concerned with precursors of diseases of older age, such as dementia; one focuses on nutrition and functional capacity; one is concemed with physical activity, food consumption, psychological factors and health, and one is designed to evaluate mutuality in social relationships, life events, and their impact on social well-being. Most of these latter studies are planned on the financially safe side, i.e., for a short term, with only two measurement times included. Furthermore, each study's coverage of the process of aging is incomplete. Recently, the Dutch government launched a program to establish more cohesion between various research initiatives and to integrate aging research more firmly into the structure of scientific research institutions (NESTOR: Netherlands Programme for Research on Aging 1989). This program should help achieve the continuity that is so essential for longitudinal research. In addition, the Dutch government has contracted out a comprehensive study of change in autonomous functioning which should cover at least ten years. This study should provide an empirical basis for policy making. In the European context, Dutch researchers play roles in the efforts to standardize measurements on aspects related to aging, such as immunology

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(EURAGE), nutrition (EURONUT), dementia (EURODEM), and health and social services use (ACRE). These efforts should improve comparability and generalizability of study findings. Sampling and Data Collection in the Netherlands: The General Practitioner One typical aspect related to sampling and data collection in which the Netherlands may differ from other countries is the central position of the general practitioner (GP). Virtually all older persons have a personal general practitioner. This GP functions as a kind of gateway between the older person and health and social services. Therefore, when an older person is selected into the sample for a health related study, the GP should at least know about it and may have to be consulted. In the DLSE, advantage was taken of this circumstance by soliciting actual collaboration from GPs. The volunteering GPs, 9% of all Dutch GPs at the time, agreed to examine 10 older persons randomly selected from their practice. Despite the existence of an excellent sampling frame provided by the population registries of Dutch municipalities, the method of recruiting subjects through GPs has been adopted in several recently started studies. This approach clearly has the advantage that sample members are approached by a person with authority whom most of them know well. They are likely to be willing to participate sooner. In addition, this procedure facilitates the collection of more objective information than would be possible with lay interviewers. Moreover, information on health care use and, as subjects die, on circumstances and causes of death can be obtained directly from the GP without having to resort to extemal data sources. On the other hand, as a consequence of this method of recruiting subjects, those elderly residing in nursing homes and hospitals are not covered by the study when they are no longer the responsibility of a GP. Their percentage is estimated to be 3%. The consequences for the attrition rate are substantial; if the study is continued longitudinally, a number of GPs may not be able or may not agree to collaborate in a follow-up examination. Also, sample members who change GP are at risk of being lost to follow-up. However, if the study is planned to be longitudinal from the start, such problems can be foreseen and provisions made to track sample members, e.g., through population registries. In the DLSE, even though this study was not planned to be longitudinal from the start, a great number of GPs and other physicians were found to be willing to examine sample members who formerly were the responsibility of GPs participating in the study. After six follow-up examinations, the number of physicians who had contributed to the study had more than doubled. The majority of sample members whose causes of death were not obtained from participating physicians were traced through municipal registries, and their causes of death were obtained from the Netherlands Central Bureau of Statistics (Deeg, Van Zonneveld, Van der Maas, and Habbema 1985).

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DORLYJ.H. DEEGAND GERARDH. VAN DER ZANDEN

International Longitudinal Studies of Aging in Historical Perspective How do the Dutch studies fit in with longitudinal studies of older persons conducted abroad? Approximately 30 foreign studies satisfy the inclusion criteria above. Space does not allow inclusion of studies focusing both on older age and earlier parts of the adult life span. 3 Seven studies started before 1960, i.e., before the computer era. These can be considered the pioneers among the longitudinal studies of aging. The Dutch Longitudinal Study among the Elderly (DLSE) belongs to these. Like DLSE, two of the studies lasted two decades or longer: the Longitudinal Study of Aging Twins (New York) and the Duke Longitudinal Study of Aging I (U.S.: Duke I). Two studies had a wide scope, although not a large number of subjects: Duke I and the U.S. National Institute of Mental Health Study. Three studies focused mainly on cognitive and psychiatric aspects: Aging Twins, the Hamburg Study (F.R.G.), and the Langley-Porter Studies in Normal and Abnormal Aging (U.S.). One study's main focus was personality, Personality Changes in Aging (U.S.), while another study, characterized by a relatively short duration and a relatively large number of subjects, focused on retirement: the Cornell Study (U.S.). The majority of these early studies drew on samples of healthy community residents or of volunteers. The decade following the "enterprising" 1950s showed less activity with respect to newly started longitudinal studies in the older age range. The initial focus of three of the five studies located reflected the general interest in antecedents and effects of retirement during the 1960s (cf. Donahue, Orbach, and Pollak 1960; Neugarten, Havighurst, Munnichs, and Thomae 1969): the Bonn Longitudinal Study of Aging (F.R.G.), the Longitudinal Retirement History Study, and the United Automobile Workers Union Retirees study (both U.S.). The first of these, however, and to some extent also the second were certainly broader in scope - a factor that may have contributed to their continuing productivity. Two other, relatively small studies, concentrated on health (the Budapest study, Hungary) and intellectual functioning (the Newcastle-uponTyne study, U.K.). In the 1970s, an increased interest in issues related to aging was reflected in the rise in the number of longitudinal studies started. In addition to the Nijmegen Relocation Project, ten studies outside of the Netherlands were located. Five studies had a prime focus on physical aspects of aging, partly stemming from a concern with prevention and partly from an interest in etiology of diseases in older age. These include the Dunedin (Florida) study, the G6teborg (Sweden) 70-year-olds study, the Konagei (Japan) study, the Older Americans Resources and Services study (U.S.: OARS), and the study of Parisian managers (France). Three other studies were also concerned with physical health, but rather from the point of view of needs arising from disability: the Manitoba (Canada) Longitudinal Study of Aging, the Massachusestts Health Care Panel Study, and the Welsh Longitudinal Study of Old Elderly. Meanwhile, all of these studies have a much wider scope, including social, psychological, and, in over half of

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the cases, cognitive aspects (Grteborg, Koganei, Manitoba, Massachusetts, OARS). Several of these studies were based on large samples, including a thousand (Koganei) or manifolds of a thousand subjects (Dunedin, Grteborg, Manitoba, Massachusetts). Two smaller studies had main foci on psychological well-being and health: the San Antonio study and the St. Louis study (both U.S.). The latter showed a special interest in cognitive function. The studies initiated in the 1970s were characterized by more representative samples as opposed to the predominance of samples composed of volunteers or healthy community residents from the earlier years. Furthermore, two studies, Grteborg and Koganei, were explicitly designed according to the cohortsequential model, which calls for the intake of new cohorts at subsequent study cycles. Discussions about interpretation problems of longitudinal data, started in the 1960s (Schaie 1965), prompted such designs. In its third cycle, the Grteborg study added a feature new to longitudinal research in the elderly: intervention. Despite the restriction of the inventory to longitudinaI studies far enough along to be found in published literature, nine studies initiated in the 1980s could already be located. To these, the Zutphen study can be added because of its renewed research efforts directed towards aging issues. Most studies draw on random or probability samples or on total populations in a geographically defined area: the Aberdeen study (Scotland), the U.S. Established Populations for the Epidemiologic Study of the Elderly (EPESE), the Kentucky Panel Study of Older Persons (U.S.), the Longitudinal Study of Aging (LSOA) of the U.S. National Center for Health Statistics and National Institute on Aging, the Melton Mowbray study (U.K.), and the South Central Connecticut study (U.S). The French Transition to Retirement study draws on samples from the National Old Age Insurance Fund for Salaried Workers. These are mostly large scale studies with 1000 to over 14,000 participants. Two smaller U.S. studies, the Utah Study of Bereaved Elderly Adults and the Washington University Memory and Aging Project, selected their samples through obituaries and physician referrals, respectively. The main focus and scope of these recently started studies are diverse. Functional capacity is an important aspect in three of the large studies: EPESE, LSOA, and Melton Mowbray. EPESE and Melton Mowbray have, in addition, a strong interest in health and cognitive functioning, while social aspects are also investigated. Transition to Retirement studies physical, psychological and social factors influencing adaptation to retirement. The Aberdeen study mainly looks into resources and health care needs. The Kentucky and South Central Connecticut studies have psychological aspects as a first interest. The smaller Utah Study of Bereaved Elderly Adults is aimed at well-being. The Washington University Memory and Aging Project, also a smaller study, has a prime interest in cognitive aspects of aging. In summary, a development can be noted from longitudinal studies concerned with a wide range of aspects of aging designed to describe normal aging and to discover mechanisms of aging, to studies designed to provide information necessary for the formulation of new policies with respect to the older popula-

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DORLYJ.H. DEEGAND GERARDH. VAN DER ZANDEN

tion and to shed light on problematic aspects of aging. PRACTICAL ASPECTS OF LONGITUDINAL RESEARCH It is not easy to do longitudinal research, especially on older persons. While it is already difficult to ensure the continuity of funding that is so essential, organizational and methodological pitfalls also loom large. Moreover, the availability of longitudinal data does not guarantee that all problems inherent in cross-sectional data can be met. The rest of this contribution discusses aspects of longitudinal research relevant to design and organization. Such aspects are not explicated in most research reports, yet they are the major determining factor of scientific output. The practical experience of investigators conducting a longitudinal study of aging can help to make a realistic assessment of such issues. In systematic interviews by the first author with investigators involved in three "case" studies (the Baltimore Longitudinal Study of Aging - BLSA, started in 1958, the Bonn Longitudinal Study of Aging - BoLSA, started in 1965, and the Established Populations for the Epidemiologic Study of the Elderly - EPESE, started in 1982; see Table II), information was collected on issues such as: what processes of change are to be studied, what are relevant endpoints, what are appropriate sampling schemes, how often and how long should follow-up examinations be conducted, how can the quality of the data be optimized, what staffing is appropriate, and what are the expected costs and returns? Those aspects related to determination of the sample, selection of topics, quality of the data, and interdisciplinary collaboration are discussed below.

Determination of Sample and Size With the exception of BLSA, the initial ages in the case studies start at 60 or 65 years. Obviously, these are the ages at which retirement takes place in most countries, and therefore "old age" starts. It is the lower limit of the realm of gerontology. However, processes of change in older age and correlates of the onset of disease may be better evaluated when earlier observations are available. In addition, a lot of migration occurs before age 65 among healthy people. Migration following widowhood and disease-related migration occur around ages 70-75. Amount and causes of migration appear to be similar across developed nations (Serow 1987). If a prospective study were to start at age 50, both migration waves would be included. The strategy to maintain equal sample sizes in each age decade is particularly useful in older populations. As sample attrition becomes greater with increasing age, the available numbers of participants decrease fastest in the highest age brackets. Thus, merely for lack of sufficient numbers, important changes that

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TABLE II Case studies: Main characteristics Study

Start End

Natureof sample

(Source: Shock et al. 1984) Baltimore 1958 cont. WellLongi- + educated tudinal c o n t . community Study intake residents of from Aging waiting list

(Source: Lehr and Thomae 1987) Bonn 1965 1983 Healthy Longicommunity tudinal residents Study W.Germany of Aging (Source: Comoni-Hunfleyet al. 1986) Esta1982 cont. Two blished (one local Popula- c o h o r t probability tions 1985) samples for the and two Epifull demiolocal logic populaStudy lations of the in US Elderly

N

Sex and ages at entry

Test Test Main interval period variables

1142 with 1st visit

M (F since 1978) 17-103

1-2 yr

2 11"2 Cfinical days genetic biochemistry work capacity special senses nutrition cognitive behavioral personality

222

M&F 60-75

1--4yr

1 week Personality intelligence soc. history well-being clinical

14500 M&F 65+

3 yr (+ annual phone interview)

1 hr+

Med.history medications func. capacity clinical hith. care use cognitive well-being social part.n demography

may take place in old age groups would otherwise be missed. The extent of oversampling depends on the attrition rates expected locally. Oversampling may also be useful in other parts of the population. In EPESE, a key role in the decision for a large sample size was played by the recognized need to examine a considerable number of Blacks. As this population group had been underrepresented in other surveys, it was not known whether their outcomes would be at great variance with outcomes of population groups more often studied. To provide for this possibility, Blacks were oversampled in one of EPESE's four locations. The sample sizes in the case studies vary from 222 (BoLSA) to 4165 (one EPESE location). It seems that the more disciplines involved in the study, the

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DORLY J.H. DEEG AND GERARD H, VAN DER ZANDEN

larger the sample size should be to allow for multivariate evaluation of aspects of aging. This is especially true for studies that are not centered around hypotheses but are rather exploratory. Although all three case studies can be characterized as exploratory in some sense, EPESE is the clearest example. Indeed, the sample sizes of these studies exceed other sample sizes by far. An interesting aspect of EPESE are the sub-studies. For these, sample members who fulfill certain criteria are selected to undergo more extensive testing. However, in some cases, initial criteria had to be "adjusted" in order to obtain minimally sufficient participants in the sub-studies. Not only did fewer sample members than expected fulfull the criteria (MacArthur Successful Aging study), but also the response was lower than in the general study (Duke Diet study). Short of abandoning the plans for certain sub-studies, a most expensive strategy to prevent this problem would be to start with a larger initial sample. Alternatively, more time could be allowed for a sub-study, in order to select substudy cases from more than one measurement point. From the literature, several aspects determining sample size are known: the desired significance level and power, the inter-individual and intra-individual variance and reliability of the instrument used for study, the anticipated magnitude of change, the number of times of measurement, and the expected attrition rate (Guyatt, Walter and Norman 1987, Overall 1987, Schlesselman 1973, St. Pierre 1980). The required sample size decreases if requirements on the significance level and power are slackened, the inter-individual variance is smaller, the reliability of the instrument is larger, the number of times of measurement is larger, and the expected attrition rate is smaller. In follow-up studies relating an exposure variable to an outcome variable (e.g., mortality), number of times of measurement and expected decline are replaced by expected number of cases (e.g., deaths) and expected effect or relative risk (Armstrong 1987, Pastemack and Shore 1981, Phillips and Pocock 1989). In this case, the required sample size is, in addition, affected by the number of categories of the exposure variable (Howe and Chiarelli 1988). In the planning stage of a longitudinal study, no full knowledge exists of all parameters involved. Particularly, knowledge of the expected magnitude of change and attrition rate will be defective. Moreover, most investigators are interested in change in more than one variable. A preliminary approach to determining, as rationally as feasible, the required sample size is to select two or three key variables and apply information concerning change and validity in each of these variables from longitudinal studies using comparable measurement instruments. Pilot studies or cross-sectional studies using comparable measurement instruments may also serve to estimate expected change. Change across age is then to replace change over time. The expected attrition rate may be derived from longitudinal studies in comparable populations or again, if need be, from pilot or cross-sectional studies. Once a study is underway, and the results are ambiguous or lack precision, it may be advisable to extend the follow-up period beyond the period originally planned. Brookmeyer, Day and Pompe-Kirn (1985) give decision rules in the

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case of an exposure-outcome study. Ambiguity of results after a certain followup time may also be anticipated before the start of a study. Pastemack and Shore (1981) propose the use of group sequential methods to calculate sample sizes in an exposure-outcome study.

Selection of Topics The first question to be answered is: How many topics should be selected? The three case studies all represent the approach of a broad coverage. As one investigator put it: "In middle age, to focus on one discipline may be adequate, but aging is a universal thing." Another investigator advocated the collection of more information than one thinks one needs "to be able to correct for one's own biases." Still, a research team may want to have a specific question answered for which a longitudinal study design is the only appropriate design. Such a question may stem from a policy need. Specific policy needs usually deal with a short-term perspective, whereas long-term policy needs are often less specific (Kars-Marshall, Spronk-Boon, and Pollemans 1988). Therefore, a longitudinal study especially designed to answer one specific policy question is likely to be too late with its answer. Other, even if not "ideal", study designs will be more appropriate in this case (cf. Kerkhoff, Keijser, Kleinendorst, and Smits 1982). The data collected based on a broad coverage approach, nevertheless, may provide answers to specific (policy) questions that were not yet conceived at the inception of the study. A recent collaboration of EPESE with the U.S. Department of Transportation is one example. To meet those critics who see the lack of clear research loci as a weakness of the study, investigators should generate a minimal number of hypotheses. This was the BoLSA strategy at later followups. Another basic question with respect to the selection of topics is: Will the main interest be in differential outcomes for groups defined by characteristics observed at baseline, or will change over time in characteristics themselves be the main focus? Both BLSA and BoLSA were very clear about this issue; their interest was in the aging process, i.e., in (intra-indlvidual) change. The studies are set up accordingly, although both studies have also dealt with comparisons of survivors with non-survivors, i.e., with the outcome survival. For EPESE, the initial interest seems less clear-cut. Whereas the initial Request For Proposal issued by the National Institute on Aging called for a representative sample that could provide (cross-sectional) descriptive information on the elderly and defined follow-up in terms of the outcomes survival, hospitalization, and institutionalization, in the course of the study alternative objectives were formulated. Thus, in more recent publications on the study objectives (e.g., Comoni-Huntley, Brock, Osffeld, Taylor, and Wallace 1986), physical and cognitive functioning is described as an additional outcome variable. The investigators now also mention the objective of finding determinants of change in functioning. Employing measures of functioning as outcome variables requires a greater

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DORLY J.H. DEEG AND GERARD H. VAN DER ZANDEN

validity of such measures. For instance, a self-report on physical functioning may become increasingly "optimistic" with increasing age, depending on the local culture. To deal with the problem of measurement invalidity, new objective physical performance measures were introduced in EPESE. These are also expected to differentiate better among those persons functioning well, to be more sensitive to change, implying decline as well as improvement, and to be a better basis for cross-national comparison (Guralnik, Branch, Cummings, and Curb 1989). The final selection of topics may not be determined solely by substantive considerations, but also by the information available from external sources. Each of the three case studies made use to some extent of external information. In BoLSA and BLSA such information is used mainly to check internally obtained information (housing conditions, causes of death); alternately, in EPESE the use of external data as supplementation to internally collected data were increasingly recognized in the course of the study (hospitalization, institutionalization, causes of death). Several other longitudinal studies, after establishing the sample, merely use routinely collected, administrative data (Baldwin, Acheson, and Graham 1987). These data need to be of sufficient quality and linkable to the sample in such a way that change over time can be analyzed (Roos, Nicol, and Cageorge 1987). In addition, the summarizing of information should be flexible so that different issues can be studied. Both as a check and as a supplementation, linkage to registry data is a powerful means to enrich the data collected in the study.

Data Quality Considering the three studies chronologically, progress has been made in terms of systematically cleaning data. In BLSA every investigator is responsible for the data on his/her specialty, and no central cleaning system has been devised. Thus, the quality of the data may vary widely across specialties. Similarly, in BoLSA, each interviewer/investigator is essentially responsible for the correctness of the data. However, in a later stage some more systematic cleaning was undertaken, although this task had a low priority. In each of the EPESE studies, cleaning of the data is recognized as a high priority, and a lot of resources are spent on this task. The availability of fast computers and the sophistication of computer programming make it possible to actually build up a data tracking and cleaning system. Also, in studies with thousands of respondents, the need for tracking and data cleaning is more obvious. Another data quality aspect is the continuity of observation or measurement instruments. This continuity depends both on those who perform the observations and on the choice of instrument. With regard to the first, there are likely to be many observers in a study of moderate to large scale and of considerable duration. Investigators should plan inter-observer studies in which the variability of the observations due to changes in observers can be estimated (cf. Rinder, Roupe, Steen, and Svanborg 1975).

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Second, even when the best contemporary measures are included in a study, new methods on measurement emerge in time. The investigators are then confronted with the question of whether the new method should or should not be adopted. In BLSA several new instruments have replaced old ones. BLSA, with its ongoing recruitment of new sample members, has an advantage over fixedcohort members in this context; it is worthwhile to implement a new technique, because as many subjects will be studied in the future as have been studied in the past. By contrast, for a fixed-cohort study, with increasing attrition, the implementation of a new instrument is questionable as the new measurements will only be available for a subgroup of survivors. The issue of measurement continuity is even more urgent in collaborative research. Calibration of the new instrument is essential. In BLSA this is done according to a procedure by which both the old and the new instrument are applied to a subsample during a few measurement cycles, and the old measurements are adjusted to the level of the new. The extent to which adjustments should be made, however, is somewhat arbitrary and depends on the importance attached to various characteristics of the distributions of both measurement instruments. No matter how important it is to master the techniques of data quality control, in the end the quality of the data hinges on the dedication of the individuals responsible for it. As one investigator emphasized, the coordinators of field work and data management should be persons who view their task as "their life's work".

Interdisciplinary Collaboration A multidisciplinary, collaborative study can not be run by one person only. This awareness leads to the recognition that an organizational structure should be created for interdisciplinary collaboration. As several investigators admitted, such collaboration contains a built-in tension. Each researcher is inclined to view other disciplines as auxiliary to his/her own; this results in differences in emphasis that permeate every phase of a longitudinal study. In cross-national collaboration, national myopia or even patriotic sentiments may compound these differences. The creation of a common "language" is the first step necessary to overcome these differences. A common "language" is based on the recognition of common interests. Several investigators emphasize the importance of cultivating such common interests in the planning pha~e of a study. Consensus may be reached by seminars where topics related to the planned study can be discussed without the constraint that decisions need to be made about practical aspects of the study. Although interdisciplinary team discussions can be time consuming because of "language" difficulties, they can also be rewarding and stimulating, giving rise to approaches or hypotheses never thought of before (Busse and Maddox 1985, Prahl-Andersen, Kowalski, and Heydendael 1979). It is important to make such positive effects visible to all investigators involved. As the study evolves, the thinking about the concepts initially underlying the

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study design may evolve as well. In EPESE, for example, a transition from outcome-oriented thinking to thinking in terms of change in functioning took place. Such transitions, however, are not necessarily shared by all investigators involved in the study. In order to keep the team together, there should be sufficient latitude to allow the co-existence of different views. Once more, the broader the scope of the study, and the more flexible its design, the better such co-existence can be tolerated. CONCLUSION Regardless of the scope and design of different longitudinal studies, which vary greatly, similar organizational, methodological, and conceptual problems are sure to emerge in their course. In this contribution some of these problems pertaining to sampling, data collection, a n d collaboration are highlighted. Continuity of sample, measurement instrument, and staff is emphasized as crucial to a study's viability, whereas shifts in research focus are shown to be signs of a study's vitality. It is hoped that this review and its recommendations will be instrumental in the initiation of longitudinal research which should provide important insights in the processes of change in older age. NOTES Paper presented in part at the XIV International Congress of Gerontology, Acapulco, Mexico, June 18-23, 1989. 2 This study was made possible by a grant from the Netherlands Ministry of Welfare, Health, and Cultural Affairs. 3 For an overview of these and for references to publications from the studies discussed here, see Deeg (1989). REFERENCES CITED Armstrong, B. 1987 A Simple Estimator of Minimim Detectable Relative Risk, Sample Size, or Power in Cohort Studies. American Journal of Epidemiology 126:356-358. Baldwin, J.A., E.D. Acheson, and W.J. Graham 1987 Textbook of Medical Record Linkage. Oxford: Oxford University Press. Brookmeyer, R., N. Day, and V. Pompe-Kirn 1985 Assessing the Impact of Additional Follow-Up in Cohort Studies. American Journal of Epidemiology 121:611--619. Busse, E.W. and G.L. Maddox 1985 The Duke Longitudinal Studies of Normal Aging 1955-1980. Overview of History, Design and Findings. New York: Springer Publishing Co. Coleman, P.G. 1976 Psychological Measures of Well-Being and Predictors of WellBeing. Nijmegen, The Netherlands: Gerontologisch Centrum. Cornoni-Huntley, J., D.B. Brock, A.M. Osffeld, J.O. Taylor, and R.B. Wallace, eds. 1986 Established Populations for Epidemiologic Studies of the Elderly: Resource Data Book. NIH Publication No. 86-2443. Washington, DC: U.S. Government Printing Office. Deeg, D.J.H. 1983 Geselecteerde bibliografie longitudinaal onderzoek op sociaalmedisch gebied. [Selected Bibliography of Longitudinal Research in the SocioMedical Field.] Rotterdam, The Netherlands: Instituut Maatschappelijke

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Gezondheidszorg, Erasmus Universiteit Rotterdam. Deeg, D.J.H., R.J. van Zonneveld, P.J. van der Maas, and J.D.F. Habbema 1985 Levensverwachting en lichamelijke, psychische en sociale kenmerken bij bejaarden. [Life Expectancy and Physical, Mental and Social Characteristics in the Elderly.] Rotterdam, The Netherlands: Instituut Maatschappelijke Gezondheidszorg, Erasmus Universiteit Rotterdam. (Summary in English) Deeg, D.J.H., R.J. van Zonneveld, P.J. van der Maas, and J.D.F. Habbema 1989 Medical and Social Predictors of Longevity in the Elderly: Total Predictive Value and Interdependence. Social Science and Medicine 29:1271-1280. Deeg, D.J.H. 1989 Experiences from Longitudinal Studies of Aging. Conceptualization, Organization, and Output. NIG Trendstudies, No. 3. Nijmegen, The Netherlands: Nedeflands Instituut voor Gerontologie. Donahue, W., H.L. Orbach, and O. Pollak 1960 Retirement: The Emerging Social Pattern. In Handbook of Social Gerontology. C. Tibbitts, ed. Pp. 330--406. Chicago: University of Chicago Press. Guralnik, J.M., L.G. Branch, S.R. Cummings, and J.D. Curb 1989 Physical Performance Measures in Aging Research. Journal of Gerontology 44:M141-146. Guyatt, G., S. Waiter, and G. Norman 1987 Measuring Change over Time: Assessing the Usefulness of Evaluative Instruments. Journal of Chronic Diseases 40:171-178. Howe, G.R. and A.M. Chiarelli 1988 Methodological Issues in Cohort Studies II: Power Calculations. International Journal of Epidemiology 17:46 A, A.68. Kars-Marshall, C., Y.W. Spronk-Boon, and M.C. Pollemans 1988 National Health Interview Surveys for Health Care Policy. Social Science and Medicine 26:223-233. Kerkhoff, W.H.C., K. Keijser, B. Kleinendorst, and P. Smits 1982 Longitudinaal onderzoek in de gerontologische beleidssfeer. Literatuurstudie in opdracht van het Nederlands Instituut voor Gerontologie te Nijmegen. [Longitudinal Research in the Realm of Gerontologic Policy. A study of literature contracted by the Netherlands Institute for Gerontology in Nijmegen.] Amsterdam, The Netherlands: Instituut voor Sociale en Bedrijfspsychologie. Kromhout, D. 1983 Body Weight, Diet, and Serum Cholesterol in 871 Middle-Aged Men during 10 Years of Follow-Up (the Zutphen Study). American Journal of Clinical Nutrition 38:591-598. Kromhout, D., A. Nissinen, A. Menotti, B. Bloemberg, J. Pekkanen, and S. Giampaoli 1990 Total and HDL Cholesterol and Their Correlates in Elderly Men in Finland, Italy and the Netherlands. American Journal of Epidemiology 131:855-863. Kromhout, D., and G.L. Obermann-de Boer 1986 Epidemiologisch onderzoek naar voedselconsumptie, leefgewoonten en chronische ziekten. [The Epidemiological Study of Food Intake, Life Style, and Chronic Diseases.] Leiden, The Netherlands: Instituut Sociale Geneeskunde, Rijksuniversiteit Leiden, Lehr, U. and H. Thomae, eds. 1987 Formen seelischen Alterns. Ergebnisse der Bonner Gerontologischen L~ingsschnittstudie (BoLSA). [Patterns of Psychological Aging. Findings from the Bonn Longitudinal Study of Aging (BoLSA).] Stuttgart: Ferdinand Enke Verlag. (Summary in English) Mednick, S.A. and A.E. Baert, eds. 1981 Prospective Longitudinal Research: An Empirical Basis for the Primary Prevention of Psychosocial Disorders. Oxford: Oxford University Press. Mertens, F. 1988 Overleven de fitsten? Psychos0ciale en demografische determinanten van levensduur uit een 12-jarig longitudinaal onderzoek rond verhuizing bij ouderen. [Do the Fittest Survive? Psychosocial and Demographic Predictors of Longevity of Older People in a 12 Year Longitudinal Study on Relocation.] Tijdschrift voor Gerontologie en Geriatrie 19:153-162. Migdal, S., R.P. Abeles, and L.R. Sherrod 1981 An Inventory of Longitudinal Studies of Middle and Old Age. New York, NY: Social Science Research Council. Nesselroade, J.R., S.M. Stigler, and P.B. Baltes 1980 Regression Toward the Mean and

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the Study of Change. Psychological Bulletin 88:622---637. NESTOR: Netherlands Programme for Research on Aging 1989 Long Term Programme. Nijmegen, The Netherlands: Netherlands Institute of Gerontology. Neugarten, B.L., R.J. Havighurst, J.M.A. Munnichs, and H. Thomae, eds. 1969 Adjustment to Retirement. Assen, The Netherlands: Van Gorcum. Overall, J.E. 1987 Estimating Sample Size for Longitudinal Studies of Age-Related Cognitive Decline. Journal of Gerontology 42:137-141. Pasternack, B.S. and R.E. Shore 1981 Sample Sizes for Group Sequential Cohort and Case-Control Study Designs. American Journal of Epidemiology 113:182-191. Phillips, A.N. and S.J. Pocock 1989 Sample Size Requirements for Prospective Studies, with Examples for Coronary Heart Disease. Journal of Clinical Epidemiology 42:639-648. Prahl-Andersen, B., C.J. Kowalski, and P. Heydendael 1979 Epilogue. In Retrospect: Recommendations for the Study of Growth and Development in the Mixed-Longitudinal, Interdisciplinary Mode. In A Mixed-Longitudinal Interdisciplinary Study of Growth and Development. B. Prahl-Andersen, C.J. Kowalski, and P. Heydendael, eds. Pp. 721-731. New York: Academic Press. Rinder, L., S. Roupe, B. Steen, and A. Svanborg 1975 Seventy-year-old people in Gothenburg: A Population Study in an Industrialized Swedish City. I. General Presentation of the Study. Acta Medica Scandinavica 198:397--407. Rogosa, D. 1988 Myths about Longitudinal Research. In Methodological Issues in Aging Research. K.W. Schaie, R.T. Campbell, W. Meredith, and S.C. Rawlings, eds. Pp. 171-209. New York: Springer Publishing Company. Roos, L.L., J.P. Nicol, and S.M. Cageorge 1987 Using Administrative Data for Longitudinal Research: Comparisons with Primary Data Collection. Journal of Chronic Diseases 40:41--49. Schaie, K.W. 1965 A General Model for the Study of Developmental Problems. Psychological Bulletin 64:91-107. Schlesselman, J.J. 1973 Planning a Longitudinal Study: I. Sample Size Determination. Journal of Chronic Diseases 26:553-560. Serow, W.J. 1987 Why the Elderly Move: Cross-National Comparisons. Research on Aging 9:582-597. Shock, N.W., R.C. Greulich, R. Andres, D. Arenberg, P.T. Costa, Jr., E.G. Lakatta, and J.D. Tobin 1984 Normal Human Aging: The Baltimore Study of Aging. NIH Publication No. 84-2450. Washington, DC: U.S. Government Printing Office. St. Pierre, R.G. 1980 Planning Longitudinal Field Studies: Considerations in Determining Sample Size. Evaluation Review 4:405-415. Van Zonneveld, R.J. 1961 The Health of the Aged. Assen, The Netherlands: Van Gorcum. Van Zonneveld, R.J., D.J.H. Deeg, M.P. van Tol, and N.P. van der Schaft-Kleywegt 1987 Nonagenarians as Survivors after 27 Years in a Longitudinal Study. In Aging - The Universal Human Experience. G.L. Maddox and E.W. Busse, eds. Pp. 318-322. New York, NY: Springer Publishing Company. Wimmers, M.F.H.G., H.J.P. Buijssen, and G.H.M. Mertens 1987 Welbevinden van ouderen na verhuizing. Gegevens van een longitudinaal onderzoek. [Well-Being of Older Persons after Relocation: Findings from a Longitudinal Study.] Nederlands Tijdschrift voor de Psychologie en haar Grensgebieden 42:36--44. (Abstract in English)

Netherlands Institute of Gerontology Nijmegen, The Netherlands