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5
CONJOINT DATA USE:
A COORDINATING COMMITTEE PERSPECTIVE
BACKGROUND
In March 19 78, the U. S . Depar tment 0 f Agr icul Lure (USDA) and the U. S .
Department of Health and Human Services (DHHS ~ submitted a proposal to
the Congressl5 for the repeated collection and systematic reporting of
information on the food, nutrition, ant health status of the U.S. popu-
lation. At the core of this proposed system, called the National
Nutrition Monitoring System (NNMS), are the USDA food consumption surveys
(the NFCS, Nationwide Foot Consumption Surveys, with its Household Food
Use and Individual Dietary Intake components) and the DHHS health status
surveys (in particular, the NHANES, National Health and Nutrition
Examination Survey, made up of nutritional and health status components).
COMMITTEE PERSPE CT IVE
A nationwide system for monitoring food consumption and nutrition,
their determinants, and their impacts must include estimates of food
acquis ition and consumption, as well as nutrient and health conditions .
These estimates are obtained from a variety of national surveys,
including the core surveys (NFCS and NHANES) and such others as the
Department of Labor Consumer Expenditure Survey, and non-HANES health
surveys. Figure 5-1 illustrates these interrelationships. Because
national food and nutrition concerns cannot be dealt with solely by the
large-scale core surveys, nationwide food and nutrition monitoring must
also include information obtained from more specific surveys -- such as
the Hispanic HANES, a nationwide health and nutrition survey of Hispanic
populations. The scope of this report is limited to recommendations for
only a part of this nationwide food and nutrition monitoring: the food
consumption and nutrient intake aspects of two national surveys, NFCS and
NHANES .
Data collection and reporting methods must meet the existing and
future needs of users of each of the separate core survey data bases,
although it is equally important that the data bases feeding information
into the larger monitoring system meet monitoring system needs, as well.
The purposes of each of the separate core surveys provide perspective on
its contributions to the overall purposes of a monitoring system. The
relation of these separate core survey data bases helps to characterize
the monitoring system and, more importantly, to define its limitations.
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PURPOSES OF CORE SURVEYS
The NFCS provides data needed to estimate amounts of foods and
nutrients in U.S. diets, as well as data useful for es timating dietary
adequacy. These estimates are reported both as aggregated data on
average comparisons and as nonaggregated data that describe intake
distributions. Survey repetition provides data that permit estimates,
within limits, of changes in foods and nutrients in U.S. diets, as well
as measurements of changes useful for estimating trends in dietary
adequacy.
The NFCS obtains information on both the kinds and the quantities of
foods. From this, estimates can be made of the nutrients in diets
ingested by survey subjects -- adults and children who are stratified
within the data bases by sex, age, and other descriptors. In addition,
these surveys obtain information on various household characteristics,
including factors that can contribute to assessment of the socioeconomic
and demographic factors associated with food consumption and dietary
status.
An essential and unique feature of the NFCS that must be preserved
and improved with high priority is the collection of data representing
multiple days of food intake by individual survey subjects. The Com-
mittee recognizes that it is this feature that, coupled wi th appropriate
statistical techniques, permits examination of the distributions of
intake among individuals. Information from either survey , sub ject to
adequate sample size, can be used to estimate average intake of defined
groups. Only the NFCS data can yield the distribution analyses needed to
estimate nutrient adequacy or food component status (see pp. 96-98~.
In comparison, the NHANES provides data needed to estimate the preva-
lence of physical states or disease conditions in the U.S. population.
Normative or descriptive data, such as data on weight and stature, are
collected. The NHANES also collects biochemical data that serve as
descriptors of biochemical and, perhaps indirectly, of phys iologic
s tatus . These biochemical and anthropometric measures can provide data
needed to estimate nutritional status of the survey population.
Successive surveys permit the repeated measurements needed to estimate,
within limits, changes over time in health and in the biochemical and
anthropometric aspects of nutritional status linked to specific nutrient
intake.
PURPOSES OF A NATIONWIDE MONITORING SYSTEM
In the view of the Committee, the purpose of a nationwide food and
nutrition monitoring system is to provide repeated collection and
effective dissemination of usable data on and descriptors of the food,
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dietary, nutritional, ant biochemical factors that contribute to the
understanding of th~nutritional status of the population. The terms in
this Statement of purpose can be described as follows:
· Repeated data collection. Similarity of measurements over time
promotes the capability of analyzing trends an well as conducting retro-
spective studies.
· Effective data dissemination. Both the diverse uses of the core
survey data and the needs of a monitoring system require the organized
and systematic management and release of data.
· Usable data. The use of varied data bases requires compati-
bility and comparability at sufficient data points to ensure necessary
linkages between data bases.
o Food and dietary factors. The system requires information not only
about food and nutrition themselves, but also about socioeconomic and demo-
graphic factors that influence the food, dietary, and nutritional status of
the population and its subgroups.
· Nutritional status. Nutritional status is a health-related judgment
based on measurements that directly or inferentially reflect the dynamic pro-
cesses of food ingestion and digestion, the transport, absorption, and metab-
olism of food components and their products, and the excretion of non-used
food components and metabol ic products .
In contrast with the use of per capita disappearance data, the advantage
of the conjoint use of NFCS and NHANES data bases for nationwide food and
nutrition monitoring is that it allows information from separate data bases to
be linked at the level of nonaggregated information. This permits data on the
dis tributions of food or nutrient intake among individuals within one popu-
lation group (e.g., identified by sex, age, and income) to be linked to other
data on similar persons in the population.
USES OF A MONITORING SYSTEM
A system that provides regular information about the population's food and
nutrient consumption with health information provides the fundamental data
needed to:
· Describe the adequacy and safety of the U.S. diet and determine
whether there are any "problems.'"
· Describe the general nutritional health status of the population and
determine if there are any '"problems."
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· Identify the population subgroups in which problems exist.
· Aid in developing strategies for resolving these problems.
· Monitor trends in nutritional status and dietary intake to determine
whether the observed prob lems are changing.
Conjoint use of data from the separate core surveys provides information
of the national scope needed to describe the status of food consumption,
nutrient intake, and nutritional health. These core survey date help to
identify national food-, diet-, or nutrient-related public concerns and their
trends along the continuum from inadequate intake to excess consumption of
specific nutrients.
A national data collection system (involving data from both the NFCS and
NINES and other data) provides the information necessary to assess the food
and nutrient-related risks to health that may be amenable to intervention.
Moreover, the system can provide information to assist in selecting among
possible options for intervention, such as the following:
· The design of food programs -- food stamp programs, supplemental food
programs, food fortification, or food guidance.
· The targeting of programs -- to population groups by sex' age,
income, or dietary pattern.
· Determination of the education content of programs -- based on the
kinds of excesses or inadequacies and their association with food and nutrient
intake patterns.
· Regulation -- setting of tolerances for substances in food or
approvals for addition of substances to food.
DATA NEEDS OF A MONITORING SYSTEM
An effective system for monitoring food and nutrition concerns will
involve other data bases, as well. For example, socioeconomic data of the
type and extent gathered by the Department of Labor and reported as the
Consumer Expenditure Survey can contribute needed perspective on questions on
the socioeconomic determinants of food patterns. Data on the composition of .
foods constitutes a separate data base that contributes fundamental infor-
mation for all other uses of the food consumption and dietary intake survey
data. The DHHS, through its National Center for Health Statistics collects
separate data base information on occurrence of diseases (morbidity), their
prevalence, and death from disease, as well as other information that
contributes to the needs of a national monitoring system. However, none of
these contributing data bases can substitute for the information obtained from
the core survey (NFCS and NHANES) data bases. As Figure 5-1 illustrates, each
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of the other data bases can contribute data needed to meet the goals of a
national monitoring~9yatem. Yet, without the information currently obtained
from the NFCS and NHANES data bases, a monitoring sys tem cannot exist .
Given the Scope of the public concerns to be addressed through nationwide
food and nutrition monitoring, neither the NFCS nor the NHANES can contribute
all the needed information. For some concerns, neither the NFCS nor the
NHANES alone can provide the needed information; clearly, data from both
surveys will be needed. Figure 5-2 illustrates this point, us ing dietary
observations an an initial point for assessing food and nutrition concerns
involving separate survey data bases. In like manner, Figure 5-3 illustrates
the concept of conjoint use with biochemical observations as the initial point
for assessing the food- and nutrition-related concerns. It should be noted
that in either case (assessment beginning with dietary or biochemical
observation) data from both surveys will be needed eventually.
RATIONALE FOR NOT RECOMMENDING A SINGLE DIETARY SURVEY
Many have suggested that it is redundant to collect national dietary
intake data through two separate surveys -- NFCS and NHANES. The Committee
considered this matter. One possibility would be a single dietary survey.
There would be three ways to attain such a single survey: eliminate dietary
data collection from one of the existing surveys, merge data collection
activities of the two surveys into one, or design a new survey after elim-
inating one or another (or both) of the two existing surveys. Each of these
approaches raises serious problems .
ELIMINATE DIETARY INTAKE DATA COLLECTION FROM ONE OF THE EXISTING SURVEYS
Eliminating dietary data from the NFCS is tantamount to eliminating this
survey ant losing the uses that require information collected only in the
NFCS, such as extensive socioeconomic data, food purchasing data, data on
seasonality, and replications .
E1 iminating dietary data from the NHANES would reduce interview time by
about 15%, but would not diminish NHANES cos ts proportionately, because NHANES
fixes costs are high. This elimination would not be desirable, because some
current and important emerging uses for the entire NHANES data base --
including use of the biochemical, clinical, and anthropometric data (which
were not a subject of this Committee's mandate) -- depend on the dietary
information in the NHANES.
Furthermore, there are other important uses that require linkages between
the detailed socioeconomic data collected through the Household Food Use and
Individual Dietary Intake components of the NFCS and the physiologic and
health measures of the NHANES. These linkages can be established either
through personal descriptors or through the dietary data, themselves. Dietary
data are required for both these linkages:
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· Dietary data are needed from the NHANES to confirm that groups that
are defines by their personal descriptors (age, sex, income, etc. ) and that
are apparently similar in their personal characteristics are indeed similar in
their dietary intake before interpretive conclus ions can be drawn across the
two surveys. This use links the survey data through personal descriptors.
· Another important link is through the dietary information itsel f, in
that groups with similar dietary intake are examined for their socioeconomic
characteristics in the NFCS and their health and nutritional characteristics
in the NHANES. This requires the same dietary information in the NHANES and
the NFCS.
MERGE THE DATA COLLECTION ACTIVITIES OF THE TWO SURVEYS INTO ONE
The NFCS is a less clustered survey sample, because it is an in-home
process; the NHANES, on the other hand, must be clustered around its clinical
examination units. Thus, NHANES data collection cannot be added to the NFCS.
If NFCS data collection is imposed on the NHANES, if would substitute the
NFCS dietary interview, adding 2 hours of dietary data collection to the
present NHANES half-hour interview and thus make the NHANES a 5-hour process.
This substantially greater respondent burden would be likely to affect
response rates adversely.
NHANES examination response rates are adversely affected by bad weather.
Therefore, the NHANES moves north and south with the seasons. This would
preclude the NFCS monitoring of seasonal patterns, because season and geo-
graphic area are inextricably confounded in the NHANES.
Because of the greater clustering in the NHANES than in the NFCS, the
merged survey would have to interview more (probably at least twice as many3
people than does the present NFCS to obtain the present precis ion of NFCS mean
estimates. Thus, a single, merged survey would probably cost more than the
two separate surveys. The exact increase in costs cannot be estimated at
present, because the necessary calculations cannot be made without NFCS
cluster identifiers, which are not now recorded in NFCS data.
The Committee recommends that these cluster
identifiers be retained in the future and that the
appropriate cost analyses be done.
DESIGN A NEW (SINGLE) SURVEY
From the above discussions, it is clear that the surveys, because they
have different purposes and uses, cannot be merged into a single entity with-
out the risk of compromising some of the fundamental purposes of one or the
other. In addition, it is unlikely that any single survey can be designed to
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satis fy all the needs for survey tata on foot consumpt ion and dietary intake
without being much more expensive than the present surveys.
These considerations and the information obtained from the users' con-
ferences let the Committee to believe that a more appropriate course was to
enumerate a series of technical recommendations to improve comparability and
compatibility between the separate surveys at currently existing points of
potential linkage. Such a course could meet the users' highest-priority needs
for more timely information and better information about who is eating what.
While at the same time accepting the realities of continuing independent
but mutually compatible surveys, the Committee recognizes the merit of con-
tinuing consideration of the desired features of a composite single survey.
To this end, the Committee developed a systematic approach to examining issues
of definition of data needed and of requisite precision and reliability of
data (Appendix A). The approach was used in the workshop. As a result,
certain issues became apparent with regard to most purposes (e.g., documen-
tation, timeliness) and other issues were more use-specific (e.g., required
quality of measures and precis ion of estimates).
The Committee found its approach (Appendix A) useful and urges that it be
made an on-going process in the continuing review of surveys, their purposes,
and their design. The Committee does not preclude the possibility that, at
some future time and with explicit statement of priority uses, a single survey
could be designed and implemented. The Committee notes only that this should
not be expected to serve the present scope of uses reported by participants in
the workshop and symposium.
NEEDS OF DATA USERS
As indicated in Table 4-2, some needs for data are common to all data
users. Thus, in the view of the Committee, recommendations for modifications
to the existing surveys that increase the utility of data developed from these
fundamental questions would have high priority. For maximal effectiveness,
the data should fulfill the needs of both the core surveys and nationwide food
and nutrition monitoring.
Each of the questions shown in Table 4-2 identifies data collection needs
and implicitly, therefore, aspects of survey use worthy of review. TheThe
data uses and data needs common to most data users, to the separate core
.
surveys and to nationwide food and nutrition monitoring were the basis for the
Committee's assessment of the highest priority options for modifying the
existing surveys.
In the view of the Committee, an improved data collection system is needed
to ensure the effective use of the separate survey data bases that are needed
for a system of nationwide food and nutrition monitoring. However, it is
equally important to bear in mind that each of the separate core surveys has
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uses that are independent of its contributions of data for nationwide food and
nutrition monitoring.
Essential independent survey purposes and uses for the data from the
separate core surveys must not be compromised by modifications intended solely
to increase nationwide food and nutrition monitoring capabilities. Rather,
des ign implications that evolve from common data uses ant needs implicit in
the priority questions of who is eating what form the basis for Committee
recommendations for modifications to the existing food consumption and dietary
intake surveys . The specific recommendations of the Committee are dis cussed
in detail in Chapters 6 and 7.
SU=ARY
The National Nutrition Monitoring System (NNMS) proposes to collect and
report data that permit estimates of the food, nutrition, and health status of
the U.S. population. At the core of this system are separate surveys: the
USDA's Nationwide Food Consumption Surveys (with its Household Food Use and
Individual Dietary Intake components ~ and the DHHS surveys (in particular, the
National Health and Nutrition Examination Survey ~ .
Data from separate surveys must be used to provide information of the
scope needed to assess the food, dietary, nutritional, and biochemical factors
associated with the nutritional health of the U.S. population and its sub-
groups. Comparable and compatible (linked) data from the separate core survey
data bases provide the information needed to assess the foodand nutrition-
related risks to health that may be amenable to intervention and to assist in
selecting from among possible intervention options.
Modifications of the existing surveys should ensure more effective con-
joint use of data from them without compromising their essential independent
uses. Information that addresses fundamental questions common to the needs of
all data users provides the basis for survey modifications of high priority.
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— 61 —
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Figure 5-2
Conjoint uses of data where concern to be addressed
originates in observations of dietary intake.
________~>NECS -- Identification of concern
through use of dietary data
,
NFCS -- Identification of the population
subgroup(~) of concern through
use of dietary data
/
i
NHANES -- Corroboration of
observations by use of biochemical
measures if appropriate measurements
can be identified and sample is large
enough ~
-- whether to
l
NFCS -- Selection from tag iOceC~e~C Ian alternatives |
Education
· Fortification
0 Food distribution programs
~ Regulation
NFCS -- Monitoring trends;
through succes ~ ive
surveys
NHANES -- Monitoring trends
through successive
surveys
.)
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Figure 5-3
Conjoint use of data where concern to be addresses
originates in biochemical measures.
- NH, AS -- Identification of concerns through deviant biochemical measures <
if appropriate measurements can be identified and sample is
large enough
/
NHANES --
.
~ i
.
Assessment of dietary association
on the basis of group means
NFCS -- Distribution of dietary association
in population subgroup(~) with dietary data
1
1
DECISION -- whether
~ to intervene
NFCS -- Selection from among intervention alternatives
· Education
· Fortif ication
· Foot assistance programs
· Regulation
NFCS -- Monitoring trends ~ I ~ NHANES -- Monitoring bents
through successive through successive
surveys surveys
~ .
.
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Representative terms from entire chapter:
dietary intake
