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OCR for page 94
4
Health Transitions and the Compression
of Morbidity
Public policy debates in the areas of health and medical care have
emerged in recent years around the question of whether improve-
ments in lifestyle and medical technology will delay the onset of
chronic illness and disability and result in a compression of mor-
bidity at older ages. If the incidence of chronic illness declines and
morbidity is compressed to a very brief period at the end of the life
span, there will be a growing number of healthy people at the older
ages. One set of policy issues emerges from this scenario:
What is the appropriate age for retirement?
Will a relatively smaller working population be able to pay
Social Security payroll taxes for the growing number and
proportion of the retired elderly?
Can productive employment be found for the growing number
of healthy older persons in the face of relatively high and
persistent unemployment?
What is the viability of tax credits, reverse mortgages, inde-
pendent retirement accounts (IRAs), and other saving plans
in assisting the future elderly to pay for a greater share of
their future health and long-term care costs?
Will income maintenance and social welfare programs be ad-
equate to meet the needs of the elderly with longer life ex-
pectancy?
Under another scenario relating to the compression of morbidity,
improvements in survival may postpone chronic illness and disability
to older ages, raising a different set of policy questions:
94
OCR for page 95
HEALTH TRANSITIONS AND THE COMPRESSION OF MORBIDIlrY 95
.
Will medical and social services be available to maintain the
independence at home of the growing number of chronically
ill and functionally disabled elderly people, avoiding ~nstitu-
tionalization?
Can we develop various types of supportive living arrange-
ments for the growing number of women age 75 and older
with no spouse who live alone?
Will public and private income support and medical care pro-
grams be adequate to meet the needs of the growing number
of disabled persons?
What type of rehabilitative programs should be designed and
made available for physically impaired elderly populations?
In assessing health transitions in old and very old populations,
a number of conceptual, measurement, and analytic issues emerge
that are not as evident in younger populations. These issues emerge
for a number of reasons.
First, and perhaps most important, is the different nature of
disease, disease interactions, and physiology in older persons. These
concerns focus on the nature of intraindividual changes in health at
later ages. There are numerous age-related changes in the physiolog-
ical state of the individual, for example, changes in metabolism, im-
mune response, and organ function, a greater likelihood of accumulat-
ing significant adverse environmental exposures, and the cumulative
effects of multiple chronic conditions and diseases. Intraindividual
changes in health at later ages involve these age-related changes and
the effect of such changes on the manifestations of specific disease
processes.
Intrinsic to these concerns is the fact that the chronic diseases
and conditions tend to manifest a different time scale than acute
disease processes more typical at younger ages. In particular, the
period of time over which chronic diseases become manifest in the
elderly tends to become significant with respect to the total life span
and to the amount of life expectancy at later ages. This greater
period of time means that, even if the incidence rates of chronic
disease were unchanged with age, there would be a greater probability
that older individuals would accumulate multiple diseases, and the
time over which the older person would have these multiple (and
interacting) diseases simultaneously would be longer. Even if such
chronic diseases become manifest at younger ages, the physiological
reserves to preserve homeostasis are generally stronger at those ages,
and the duration of disease expression may be shorter with disease
OCR for page 96
96
AGING POPULATION IN THE TWENTY-FIRST CENTURY
control (if not cure) more completely and rapidly imposed by the
physiology of younger persons. Alternatively, if a chronic disease is
atypically manifest at younger ages, it may be due to particularly
powerful genetic or other susceptibilities to these disease processes,
in which case the natural course of the disease may be dramatically
shortened by loss of homeostatic control and death.
Thus, health transitions at later ages cannot be described sim-
ply in terms of the incidence rate of independent disease events.
Instead they must be viewed in terms of multiple, interacting disease
processes that operate in a host with generally declining homeo-
static reserve. It is clear that these concepts involve modeling health
changes in the elderly person as changes in a complex multidimen-
sional system evolving over a considerable period of time.
To do so requires addressing formidable measurement and ana-
lytic problems that are not well handled by our current data collection
and analytic strategies. The practical need for such data and analytic
strategies is already manifest by the underlying concepts in the de-
velopment of clinical specialties in geriatric medicine (e.g., Minaker
and Rowe, 1985; Besdine, 1984) and in concerns with the special
problems in clinically managing and treating specific diseases (e.g.,
lung cancer, breast cancer, isolated systolic hypertension, diabetes)
in older persons.
It should also be noted that there are many examples of such
complex system-based models in the physical sciences that could be
the point of departure for developing appropriate health transition
models for the elderly. The development of such models, however,
requires an approach or philosophy of inference that differs from the
approach typically employed in experimental or clinical trial designs;
that is, we wish to develop an approximate mathematical description
of a complex physical system rather than to test whether an observed
effect would have reasonably occurred simply by chance.
In addition to these differences in health states and their changes
in elderly individuals, other issues emerge in modeling the health
changes in the populations of the elderly. These arise because people
who are alive at later ages represent only a portion of their cohorts,
since significant mortality will have occurred in these cohorts"
mortality that will systematically select out persons according to
the health characteristics we wish to assess. Issues in measurement
also occur because, though disease prevalence may increase, popu-
lation prevalence (i.e., the number of persons available for study in
OCR for page 97
HEALTH TRANSITIONS AND THE COMPRESSION OFMORBIDIlrY 97
any well-defined population) at very advanced ages is small. Fur-
thermore, the description of the disease process becomes complex
and the rate of changes in physiological state may accelerate when
homeostatic mechanisms become weaker at advanced ages or due to
the accumulation of multiple disease processes. This complexity of
disease state may make even the development of crude indicators
of health state at advanced ages more difficult (e.g., the need for
multiple-cause mortality data rather than underlying-cause mortal-
ity data to describe mortality patterns among the oldest-old).
From the above discussion, it is clear that new concepts and
models that are based on process and duration rather than on event
and incidence need to be developed to assess health changes at later
ages. Indeed, the many recent debates that have emerged in the
biomedical and health policy literature about the direction of recent
health changes at later ages may be due simply to the lack of common
concepts and models. It is clear, however, that the complexity of
the physical system being described makes it difficult to have a
pars~rnonious and relatively simple conceptual model to help inform
the debate and improve concepts and communication.
In order to remedy this problem, a World Health Organization
Scientific Advisory Group (World Health Organization, 1984) de-
veloped a model of health transitions at later ages. This model is
based on simple life-table concepts in order to develop a theoretical
framework that would be readily understood at the policy level. The
basic model is actually quite simple, as shown in Figure 4.1.
Figure 4.1 contains a series of three survival curves labeled mor-
tality, disability, and morbidity. The outermost curve (mortality)
represents the effects of mortality on overall survival. We see that
the proportion surviving from a cohort to each successive age de-
clines (i.e. as one progresses to the right on the horizontal axis). In
addition to the overall survival curve, there are survival curves for
chronic morbidity and chronic disability. The lowest survival curve
(morbidity) represents the probability of surviving to a given age free
of chronic morbidity. The area labeled A under this curve represents
the number of person-years that a person in this cohort could expect
to survive free of disease (i.e., healthy life expectancy).
The middle curve, labeled disability, describes the probability
of surviving to a given age free of serious chronic disability. The
formulation assumes that disability emerges as a result of the pro-
gression of some underlying morbid process. This curve identifies
two additional areas in the figure.
OCR for page 98
98
._
.>
AGING POPULATION IN THE TWENTY-FIRST CENTURY
80
60
50
20
10
0 10
DISABILITY
MORTALITY
__ - . As_
MORBIDITY ~ . ~
A" 1-- -'
.
1 1
20 30 40 50 60
. ~
- \
. ~
\~\ ~'\~\\
%~-,\x
`- ~
80 90 100 110
`. ~
'my_
Age
70
eO ~ ~ e60
50
eo and e60 are the number of years of autonomous life expected at birth and at
age 60, respectively. Mso is the age to which 50 percent of females could expect
to survive without loss of autonomy.
FIGURE 4.1 The observed mortality and hypothetical morbidity and disabil-
ity survival curvier for females in the United States of America in 1980.
Source: World Health Organization (1984~. Reprinted by permission.
The first, the area between the morbidity and disability survival
curves labeled B. defines the number of person-years that can be
expected to be lived by an individual from the cohort in a morbid,
but disability-free, state. This portion of the figure has implications
for acute health services, but relatively little direct impact on Tong-
term care services. Many of the diseases that are fatal in middle age
may produce a significant contribution to this portion of the figure-
but relatively little to the disabled person-years to be experienced by
the cohort. Examples of this type of disease process are cancer and
myocardial infarctions.
The second area defined by the disability curve lies between it
and the mortality curve and is labeled C. This area represents the
person-years that can be expected to be lived by a person from this
cohort in a chronically morbid and disabled state. Recall that the
OCR for page 99
HEALTH IrRANSITIONS AND THE COMPRESSION OF MORBID 99
/
total area under the disability curve (areas A and B) represents
the number of person-years that can be expected to be lived in a
disability-free state. This total area may be viewed therefore as
a measure of active or autonomous life expectancy (e.g., Koizumi,
1982; Katz et al., 1983; Wilkins and Adams, 1983~. Certain dis-
ease processes will have a magnum effect on active life expectancy;
for example, onset of a serious stroke or Alzheimer's disease may
immediately produce serious disability. Such diseases have serious
consequences for long-term care as well as acute care services.
With the three survival curves, it is possible to more precisely
distinguish between the positions of different analysts about changes
in health at advanced ages. For example, a major area of debate
has centered on whether the health status of elderly persons has
improved at each age as life expectancy at later ages increased. This
debate was central to discussions about whether the entitlement age
for social security could reasonably be increased from 65 to 67 to
adjust for a two-year increase in life expectancy for males at age
65. Some analysts argued (e.g., Feldman, 1983) that life expectancy
increased because chronically ill and morbid persons were surviving
for longer periods of time. This could be represented in Figure 4.1 by
the mortality survival curve moving to the right but the morbidity
and disability curves remaining unchanged. This would increase the
absolute numbers of years expected to be spent by a population in
morbid and disabled states as well as the ratio of dependent morbid
person-years of life to the total number of person-years expected to
be lived.
The majority opinion was that as the mortality curve moved to
the right, both the morbidity and disability curves moved to the
right in a reasonably "parallel" fashion. Thus, the probability of
surviving to age 67 free of disease and disability was the same after
the recent life expectancy increases at later ages as it had been at age
65 before the increase. This suggests that the ratio of disabled and
morbid person-years to be lived to the total would decline, raising
the questions of how this improvement could be turned to society's
benefit.
These discussions relate to a topic currently of considerable in-
terest: the compression of morbidity. This concept suggests that
different public health policies will have different effects on how
rapidly each of the three curves moves to the right. For example,
focusing exclusively on treating diseases after they emerge may cause
life expectancy to increase but allow the age at onset of disease and
OCR for page 100
100
AGING POPULATION IN THE TWENTY-FIRST CENTURY
disability to remain the same, with a net increase in the number of
years lived in a morbid and disabled state. Alternatively, certain
prevention and health promotion strategies may delay the age at
onset of disease more than life expectancy Is increased. In this case,
the period spent in a morbid state and the acute health resources
required may be reduced. Interestingly, recent data suggest that
the diseases responsible for most of the disability and dependence
at advanced ages (e.g., arthritis, Alzheimer's disease) are not those
typically targeted by prevention efforts (e.g., heart disease and can-
cer). Thus, without careful attention to the effects of prevention on
disability as well as disease, the morbid period may be decreased
(decreasing acute health care costs) but the disabled period might be
relatively untouched thus having little effect on aggregate needs for
long-term care (Manton, 1986b). Alternatively, the rate of progress
of the disease may be retarded so that the age at onset of disability
may increase more than life expectancy. In this case, the disabled
period is compressed or reduced.
A specific example of the ejects of such compression was dis-
cussed by Brody (1987~. He points out that the risk of hip fracture
increases roughly exponentially with age. As a consequence, if the
age at onset curve of hip fracture could be shifted five years to the
right (i.e., the process retarded five years, perhaps by nutritional
supplementation or hormonal therapy), the prevalence of hip frac-
ture in the population would be cut in half, provided there were no
corresponding increases in life expectancy.
One extreme form of the compression of morbidity argument
(Fries, 1980, 1983) suggests that the United States and other devel-
oped countries currently have life expectancies near to their biological
limits. As a consequence, the survival curve for mortality will have
little potential to move much to the right with a corresponding rapid
increase in the prevalence of death due to biological senescence, i.e.,
natural death (Fries, 1983~. Since natural death is argued to be due
to senescent processes, it is also argued that it will be independent of
the chronic disease processes that produce disability. Thus, while the
mortality curve is fixed, the age at onset of disease and disability can
be delayed by appropriate, independent action. This will cause the
area between the survival curve for disease and disability and that
for mortality to compress until, at the extreme, the age at onset of
disease and disability is pushed beyond the biologically determined
age at death, with the result that people die of senescence free of
disease or disability.
OCR for page 101
HEALTH TRANSITIONS AND THE COMPRESSION OF MORBIDIlrY 101
One problem with the formulation of the compression of mor-
bidity mode! presented in the preceding paragraph is that it is hard
to define "natural death." Indeed, Fries (1983) has suggested that
the dividing line between natural and premature death may be an
arbitrary one. Furthermore, the available evidence on the period of
terminal decline leading to death suggests that it has not changed as
life expectancy at later ages increased, nor is it highly age-variable
(e.g., Riley et al., 1985; Roos et al., 1986~.
Eries's optimistic argument differs considerably from those of
many other investigators. Clearly it is different from the arguments
of Feldman (1983) about what we have experienced. Such arguments
do not seem to take into account that to achieve the goals set for the
adoption of health promotion and disease prevention measures, major
efforts are required in some population subgroups, in particular,
lower socioeconomic status groups and in some minority groups such
as blacks. Nonetheless, it may have utility in defining disability or
morbidity prevention goals for selected population groups.
Apart from practical concerns about the pure form of the com-
pression of morbidity model, there are those who propose very dif-
ferent mechanisms for health changes at later ages. For example,
Kramer (1981) and Gruenberg (1977) suggest that we are currently
increasing the number of years to be spent in morbid and disabled
states. This is because they view the bulk of biomedical research as
being directed toward reducing mortality and not toward decreasing
morbidity and disability. Thus, they suggest that we have effective
strategies for moving the mortality curve to the right but not for
moving the morbidity and disability curves.
StrehIer (1975), in contrast, suggests that we should intervene in
the basic aging process. He suggests that within 25 years we should
be able to increase the life span by 25 percent. He also suggests
that, since we wiB intervene in basic aging processes, the number of
years spent in morbid and disabled states will not increase. Thus, ah
three curves will move to the right an equal amount so that only the
number of person-years spent free of disease (area A) will increase,
causing a massive decrease in the ratio of impaired to healthy years.
Each of the above arguments is presented by its respective author
as a conceptual model to illustrate a particular perception of the
age-changing relation of morbidity, disability, and mortality. For
the purposes of actually forecasting likely changes in health status
and service needs, we need a more complete and feasible model to
OCR for page 102
102
AGING POPULA1TON IN THE TWENTY-FIRST CENTURY
accommodate what is being learned from recent epidemiological and
biomedical research on aging-related health changes.
The development of the more general mode! could begin by rec-
ognizing that the human organism is subject to multiple pathological
processes at later ages. Furthermore, it should be recognized that
different diseases have different associated periods of morbidity and
disability. For example, cancer has a relatively long morbid period
but a short disabled period. Myocardial infarction has short mor-
bid and disabled periods (assuming effective interventions for these
diseases exist). Alzheimer's disease has long morbid and disabled
periods. Thus, the total morbid and disabled periods for the popula-
tion can be compressed by targeting interventions to individuals at
high risk of the disease processes with the longest disabled periods
(assuming effective interventions for these diseases exist). This will
change the mix of disease processes operating in the population and
may reduce the disabled period more rapidly than life expectancy
is increased. This is not to argue that efforts should not continue
against the major fatal conditions but instead suggests that inad-
equate attention has been paid to the potential prevention of the
disease processes generating the most disability (e.g., Alzheimer's
disease, arthritis, depression). Such a strategy of accelerating com-
pression seems more plausible since it recognizes the multiplicity of
chronic conditions affecting the elderly, does not require the concept
of natural death, relates interventions to well-defined disease enti-
ties, and allows interventions in the disease processes to have effects
on life expectancy (i.e., does not require that disease and mortality
processes operate independently). It also is arguably a much better
representation of our best current scientific evidence. For example,
we have little evidence that natural death exists. Autopsy studies
of the extreme elderly do report increasing prevalence of multiple
chronic diseases at later ages, although it is admittedly more diffi-
cult to identify a single cause of death. Nonetheless, interventions
in particular disease mechanisms may help to incrementally improve
overall system performances.
Verification of results from this type of mortality data also comes
from longitudinal studies of aging populations. For example, I.akatta
(1985) has studied cardiovascular functioning in the extremely elderly
(i.e., persons in their eighties) and found that, in persons free of
disease, cardiovascular function could be preserved to advanced ages.
Similar evidence has emerged for other organ systems from studies
in which individuals with manifest pathological changes have been
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HEALTH TRANSITIONS AND THE COMPRESSION OF MORBIDITY 103
removed from the analysis. For example, Katzman (1985) discusses
the fact that cognitive functions appear to decline very slowly in
longitudinally followed populations, especially in persons not close
to death. He contrasts these results with those of cross-sectional
studies, which show more rapid declines that appear to be artifactual.
Such results are consistent with studies of cause-specific mortal-
ity in the United States, which find that the same general mix of
diseases is causing mortality at ages that are 2 to 4 years higher on
average than they were 15 to 20 years ago. It is also interesting that
studies of the variance of the age at death at advanced ages do not
show the variance decline with life expectancy increase that would
be expected if the upper tad! of the age at death distribution were
truly being truncated by an absolute biological limit to life span at
the current leveb of life expectancy.
Although a general mode! is needed, the mode! presented in Fig-
ure 4.1 is a simplified conceptualframework for studying age changes
in the relation of morbidity, disability, and mortality in a population.
It has merit because it helps us to understand the changing relation
of those health events in a simple framework. Actual analyses will
be more complex dealing with the multiplicity of chronic diseases at
advanced ages, the possibility of reversals in chronic morbidity and
disability, dependent competing risks, the dynamics of human aging
processes and risk factors, systematic mortality selection, and the
fact that there is a many-to-one mapping between chronic diseases
and the type and level of disability.
In addition to analytical issues, the mode] also raises questions
about the nature of data that are required to evaluate the transition.
In its simplest form, the mode} can be evaluated using population life
tables, surveys of disability, and epidemiological studies of disease in-
cidence. For example, Wilkins and Adams (1983) used the essence of
the concept in assessing active life expectancy gains in the Canadian
population. They found that of a Year gain in total life expectancy
between 1951 and 1978, 4.7 years was in an activity-limited state.
Recent life insurance studies suggest that the level of activity lirni-
tation was quite low (Blanchet, 1986~. More detailed models would
involve specific disease processes and their differential contributions
to the years lived in each state. Thus, the general concept of the
models can help guide us both in terms of needed data and analytic
strategies.
#
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104
AGING POPULATION IN THE TWENTY-FIRST CENTURY
DATA NEEDS AND RECOMMENDATIONS
The statistical methodologies and analyses reviewed in this chap-
ter also serve the purpose of identifying critical data needs. In order
to describe health transitions at advanced ages, certain specific types
of new data are needed and the development of new methodologies
for analyzing longitudinal data is required.
One need is for long-term follow-up of well-characterized co-
horts. In order to describe health transitions among the elderly it
is necessary to have repeated measurements of detailed physiolog-
ical, social, and health characteristics at advanced ages (i.e., over
753. Such follow-up data would help us to understand the dynamics
of physiological aging processes. For example, we might be able to
determine whether certain risk factors such as socioeconomic sta-
tus and overweight cease to be so strongly associated with disease
risks at advanced ages, e.g., such follow-up data are necessary to
help us disentangle the effects of systematic mortality selection from
basic physiological aging processes. A cost-effective approach for
generating such measurements at advanced ages would be to make
such measurements of existing longitudinal study populations. It
is recognized that a full application of the concept advanced may
not be feasible, but a limited set of items on physiological and psy-
chosocial functioning could be incorporated in local and national
studies. Existing surveys, especially local and national pane} stud-
ies, can be enhanced by repeated measurement of physiological and
psychosocial functioning, capitalizing on the probability sample and
baseline measurements available. For example, specific investigation
of system functioning, such as cardiovascular function, can benefit
by adding studies to existing panel studies with probability samples.
Population estimates of physiological parameters at baseline and un-
der conditions of stress (such as a treadmill test) can be derived from
such add-on studies to existing cohorts. These data can also be user]
in longitudinal predictive analyses to enhance usual demographic and
self-reported data.
Recommendation 4.1: The pane! recommends that detailed
physiological assessments be added to existing longitudinal
study populations when these cohorts can serve as an appro-
priate sample to measure specific biological and psychosocial
functioning (e.g., cardiovascular status and capacity for daily
activities).
Consideration needs to be given to the addition of physiological
~ ~. - r ~ 1
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HEALTH TRANSITIONS AND THE COMPRESSION OF MORBIDITY 105
assessments to the Longitudinal Study on Aging and to community-
based special surveys. Examples of community-based epidemiological
studies for which follow-up might be done are the Evans County
(Cornoni-HuntIey et al., 1986) and Alameda County (Berkman and
BresIow, 1983) studies. Nationally representative survey populations
that might be considered are those in the Longitudinal Retirement
History Survey and the National Long-Term Care Study.
A second need is to determine morbidity and mortality risks
among nationally representative samples of already morbid and dis-
abled persons. This would help us resolve some of the questions
about the effect on society of life expectancy gains at later ages. The
prevalence of many chronic diseases is too low to permit the analysis
of morbidity and mortality risks for morbid subpopulations using
general health surveys. Adequate samples of morbid subpopulations
can be obtained by use of a two-stage design in which a large popula-
tion is screened in the first stage to identify individuals with certain
chronic diseases a target group followed by a second phase of in-
tensive interviewing of the target group. For example, in 1982 and
1984 a survey was conducted of chronically disabled elderly persons.
In that survey persons drawn from the National Health Insurance
Master file were screened by telephone to determine if they had a
chronic (more than 90 days) ADL or lADL disability. If the answer
was yes, then an intensive household survey was done. Screening
approximately 36,000 persons identified 6,393 disabled persons. This
design provided a cost-effective way to define a disabled subsample
that could be followed for future morbidity changes and mortality.
Sample members were followed for two years to examine their risk of
death, their risk of institutionalization, and the trajectory of change
in dependency level. Such data are important in forecasting future
health service needs" forecasts that are essential to projecting the
costs and resource requirements for health care of the elderly. These
projections are required in considering funding and organization of
health care.
Recommendation d.2: The pane! recommends that spe-
cially focused studies and surveys of specific chronically mor-
bid and disabled elderly populations be conducted.
A third type of useful data could be obtained by systematically
extending ciata collection for clinical studies to measure comorbidi-
ties (i.e., multiple morbid conditions). Not only is the issue of co-
morbidity important for describing normal aging changes, but it is
OCR for page 106
106
AGING POPULATION IN THE TWENTY-FIRST CENTURY
also important for determining what are the appropriate clinical re-
sponses to disease at advanced ages. Currently most clinical data
bases do not collect extensive data on comorbid conditions. Such
data are important for assessing age-related differences in response
to different types of medical and surgical treatments.
Recommendation 4.3: The pane] recommends that stan-
dard guidelines be developed for collecting clinical data on
comorbidities at advanced ages and that the National Center
for Health Statistics and various institutes of the National
Institutes of Health promote the collection of such data in
regularly funded clinical study programs.
A fourth requirement is for data that systematically relate chron-
ic disease diagnoses to level and type of disability. This would allow
us to generate the mapping functions of diseases to disabilities and
to utilize the extensive epidemiological data on disease risk factor
associations in forecasting future demand for Tong-term care services.
Recommendation 4.4: The pane! recommends that stan-
dard health surveys and epidem~ological studies include ques-
tions that help clarify the relationship between different dis-
eases and different types and levels of disability at different
advanced ages.
Specifically, questions should be included that identify the diseases
that are the major causes of different components of functional loss
and disability.
A fifth requirement is for increased geographic disaggregation of
data. Geographic disaggregation would allow us to take better advan-
tage of naturally occurring population differences in risk. Different
subpopulations have very different exposures and disease risks. Such
disaggregation would require more detailed tabulation of currently
existing data. It is therefore a relatively low-cost way of increasing
the usefulness of data that have been collected.
Recommendation 4.5: The pane} recommends that more
studies and analyses be conducted of the geographic varia-
tion of morbidity and mortality risks.
In order to be able to conduct such studies, it will be necessary to
examine the question of identifying specific morbidity and mortality
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HEALTH TRANSITIONS AND THE COMPRESSION OF MORBIDITY 107
events at a level of aggregation detailed enough that the area pop-
ulations studied are relatively homogeneous. County-level mortality
data need to be disaggregated to smaller areas.
Probably no single data source is adequate to study detailed
health transitions at advanced ages. The interactions of chronic dis-
eases ax, 8 aging changes are such complex phenomena that they can
best be studied by using information from multiple data sources.
This suggests the need for biologically motivated models of the ba-
sic physiological processes that can be used to integrate parameter
estimates developed from different data sources. Such a strategy
for integrating and evaluating multiple data sources must be pur-
sued both as a way of systematizing our knowledge about health
dynamics and as a strategy for identifying critical data needs.
Recommendation 4.6: The panel recommends that the Na-
tional Institute of Aging and the National Science Founda-
tion conduct or sponsor studies to develop statistical pro-
cedures that can utilize multiple data sources for studies of
health transitions.
Representative terms from entire chapter:
advanced ages
