NHANES Dietary Web Tutorial: Dietary Data Overview: History of Diet... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/DietaryD...
Task 1: Key Concepts About the History of Dietary Data Collection
Federal agencies have collected food consumption data for decades. These data are used for a variety of policy making and research purposes in
the areas of nutrition monitoring and surveillance, dietary guidance, food fortification, environmental exposure assessment, and nutrition assistance
and education program planning and evaluation.
USDA Data Collection
The U.S. Department of Agriculture (USDA) has collected national food consumption data for more than 70 years. Initially designed to help people
achieve economical and nutritious diets, USDA's food consumption surveys gradually broadened in scope and purpose. Nationwide surveys were
conducted in 1965-1966, 1977-1978, 1985-1986, 1987-1988, 1989-1991, 1994-1996, and 1998.
The most recent surveys, called the Continuing Survey of Food Intakes by Individuals (CSFII) and conducted in 1989-1991, 1994-1996, and 1998,
were combined with USDA's Diet and Health Knowledge Survey (DHKS), a telephone survey designed to measure knowledge and attitudes about
diet and health. Table 1 provides an historical overview of USDA's nationwide food surveys.
Table 1. Overview of USDA Nationwide Food Surveys, 1936–19981,2 (click to view)
HHS Data Collection
In 1971, the U.S. Department of Health and Human Services (HHS) added a nutrition component to its National Health Examination Survey (NHES)
and the survey's name was changed to the National Health and Nutrition Examination Survey (NHANES). Between 1971 and 1994, three NHANES
collected dietary data along with health data (1971-1975, 1976-1980, 1988-1994). In 1982-1984, Hispanic HANES was conducted to collect nutrition
and health data on Mexican Americans, Cuban Americans, and Puerto Ricans. Since 1999, NHANES has been a continuous, annual survey
program. Table 2 provides an historical overview of the diet-related components of NHANES.
Table 2. Summary of NHANES Diet-related Questionnaire Content and History of use in NHANES 1,2 (click to view)
Combined Data Collection
Beginning in 2002, HHS and USDA integrated their NHANES and CSFII dietary data collection efforts. Dietary intake data on the U.S. population are
now collected continuously rather than on a periodic basis as was true for previous NHANES and CSFII surveys. HHS and USDA jointly plan the
dietary interview, which is used to collect 24-hour recall data. The dietary recall interview component is called What We Eat in America (WWEIA).
USDA has primary responsibility for the development and maintenance of the automated dietary interview system, dietary data processing, and
maintenance of the technical databases that are used to report the dietary intake findings. (For more information about USDA dietary data, see the
"Resources for Dietary Data Analysis" module later in this tutorial.) HHS is responsible for the NHANES survey sampling methodology and data
collection activities. The integration of the two survey systems has allowed the agencies to link diet information to NHANES health information, thus
providing a rich resource of information to investigators and policy-makers.
IMPORTANT NOTE
For more information about the history of Dietary Data collection by USDA and NHANES and the integrated system, see the Related References
page.
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NHANES Dietary Web Tutorial: Dietary Data Overview: NHANES Diet... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/DietaryD...
Task 2: Key Concepts About NHANES Dietary Data Collection
NHANES Data Collection
NHANES collects data using interviews, physical examinations, and questionnaires:
All participants complete an in-person household interview.
Following the household interview, all survey participants are asked to complete a health examination. Examinations are performed in a Mobile
Examination Center (MEC). The health examination component consists of clinical tests, laboratory studies, and additional interviews. Some
tests are only administered to select sub-samples.
Following the health examination, participants may complete additional interviews and questionnaires. The post-examination interviews and
questionnaires are administered by telephone or mail. Not all questionnaires are administered to all participants.
NHANES Dietary Data Collection
Three major types of dietary data are collected in NHANES:
dietary behavior
24-hour dietary recall, and
food frequency questionnaire.
Dietary Behavior
During the household interview, information is obtained on dietary behavior including topics such as dietary modifications due to health conditions,
and dietary supplement use. Demographic, socioeconomic and other health-related questions are also administered.
24-hour Recalls
During the MEC exam, a 24-hour dietary recall is administered in addition to medical and dental examinations, physiological measurements and
laboratory tests. The 24-hour recalls are conducted in English or Spanish. These dietary interviews are conducted by dietary interviewers. Survey
participants 12 years and older complete the dietary interview on their own. Proxy respondents report for children who are 5 years and younger and
for other persons who cannot self-report. Proxy-assisted interviews are conducted with children 6-11 years of age. Beginning in 2002, all participants
were asked to complete a second 24-hour dietary recall (Day 2) interview. The NHANES Day 2 dietary recalls are collected by telephone
approximately 3 to 10 days after the MEC exam. Before NHANES 2002, a second 24-hour recall was collected on a small subset of participants.
The following table shows the data that are collected for the two 24-hour recalls.
Food and Participant Data Collected During the Two 24-Hour Recall
Link Description
Information specific to Detailed description (type, form, brand name)
each food and beverage Additions to the food
consumed on a recall Amount consumed
What foods were eaten in combination
day:
Time eating occasion began
Name of eating occasion
Food source (where obtained)
Whether food was eaten at home
Amounts of food energy and more than 60 nutrients/food
components provided by the amount of food (calculated)
Information specific Day of the week (recall day)
to the recall day: Amount and type of water consumed, including total plain water,
tap water, and plain carbonated water
Recall day's consumption amount compared to typical diet
Daily total intakes of food energy and more than 60
nutrients/food components (calculated)
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NHANES Dietary Web Tutorial: Dietary Data Overview: NHANES Diet... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/DietaryD...
Link Description
Information specific Source of tap water
to a participant's Added salt: Frequency and type of salt added at the table and
overall diet: when preparing food
Whether on a special diet and type of diet
Frequency of fish and shellfish consumption (past 30 days)
Interviewers administer the 24-hour recall using USDA’s Automated Multiple-Pass Method (AMPM). This figure shows how the data are collected in 5
standardized steps:
USDA Automated Multiple-Pass Method (AMPM)
Food Frequency Questionnaire
The Food Frequency Questionnaire (FFQ) was mailed to respondents following completion of the second 24-hour recall in NHANES 2003-2006. The
FFQ collects information on the frequency of consumption of foods and food groups during the previous 12 months. Additional information on types of
foods consumed, such as sugar-free soft drinks; whole grain foods; and light, low-fat, or fat-free foods, was also included. The National Cancer
Institute (NCI) developed the FFQ based on its Diet History Questionnaire (DHQ), a food frequency instrument that is widely used in nutritional
epidemiology research. The DHQ food item list was compiled using data from the 1994-1996 CSFII. The "Related References" page includes several
research articles on the development and validation of the DHQ and NHANES FFQ instruments and the uses of FFQ data in research. NHANES
participants who completed both 24-hour recalls were mailed the FFQ component. English and Spanish versions of the FFQ instrument were used.
Unlike the DHQ, portion size information was not collected. The FFQ results are not intended to be used for nutrient intake estimation, but rather to
assess usual dietary intake.
As with the 24-hour recall, survey participants 12 years and older provide their own responses for the food frequency questionnaire. Proxy
respondents report for children who are 5 years and younger and for other persons who cannot self-report; and proxy respondents assist children
6-11 years of age.
IMPORTANT NOTE
For more information about the history of Dietary Data collection by USDA and NHANES and the integrated system, see the "Related References"
page.
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NHANES Dietary Web Tutorial: Dietary Data Overview: Uses of the Data https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/DietaryD...
Task 3: Key Concepts About Uses of the Dietary Data
Statistics about the population’s dietary intakes, estimated from NHANES data, are used to monitor the nation's diet and to inform and evaluate public
policy in the areas of:
dietary guidance,
food fortification,
exposure to pesticides and other environmental contaminants, and
nutrition assistance and education.
NHANES findings have been used to establish national reference data on total population intakes of nutrients from foods and dietary supplements,
estimate usual food and nutrient intake of the U.S. population, identify food sources of essential nutrients among population subgroups, and evaluate
compliance with current recommendations for food and nutrient intakes by population subgroups, including infants, children, pregnant and lactating
women, and the elderly.
Researchers in the U.S. and around the world use NHANES dietary data to examine trends and relationships between diet and health using cross-
sectional data. NHANES participants also are tracked over time for longitudinal follow-up research studies.
Information from NHANES is featured in scientific and technical journals and reports. For data users and researchers throughout the world, survey
data are now publicly distributed on the NHANES website.
Primary data users are the HHS and USDA agencies that collaborate in designing, developing, and carrying out the survey. This partnership also
extends to other federal agencies, such as the Environmental Protection Agency. Research organizations, universities, health care providers,
educators, industry, and the general public also have benefited from the survey information in various ways.
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NHANES Dietary Web Tutorial - Dietary Data Overview https://www.cdc.gov/nchs/tutorials/dietary/SurveyOrientation/Navigate/i...
Navigate NHANES Website
Purpose
All NHANES data are available on the NHANES website, and dietary data are one of the main topics included there. In order
to analyze NHANES dietary data, it is therefore important to understand how the NHANES website is organized. This module
provides users with an orientation to the overall NHANES website.
Navigate the NHANES Homepage
By getting acquainted with the NHANES homepage, you will gain an appreciation for the number and diversity of materials
available through the NHANES website.
Key Concepts about Navigating the NHANES Homepage (/nchs/tutorials/Dietary/SurveyOrientation/Navigate/Info1.htm)
How to Navigate the NHANES Homepage (/nchs/tutorials/Dietary/SurveyOrientation/Navigate/Frame1.htm)
Task 2: Find Datasets on the NHANES Website
All publicly available data and related documentation, including those for dietary data, are released and updated on the
NHANES website. Information on accessing non-publicly released data is also available on the NHANES website.
Key Concepts about Finding Datasets on the NHANES Website (/nchs/tutorials/Dietary/SurveyOrientation/Navigate
/Info2.htm)
How to Find Publicly Released Datasets and Information on Non-publicly Released Datasets (/nchs/tutorials/Dietary
/SurveyOrientation/Navigate/Frame2.htm)
Task 3: Find Survey Background Information
Reviewing the NHANES survey background information is an important step as you determine the scope of your analysis and
which variables to include.
Key Concepts about Finding Survey Background Information (/nchs/tutorials/Dietary/SurveyOrientation/Navigate/Info3.htm)
How to Find Survey Background Information (/nchs/tutorials/Dietary/SurveyOrientation/Navigate/Frame3.htm)
Task 4: Find the Analytic and Reporting Guidelines
The Analytic and Reporting Guidelines should be used for NHANES data analyses and publications. The guidelines provide
users a framework for producing estimates that conform to the analytic design of the survey.
Key Concepts about Finding the Analytic and Reporting Guidelines (/nchs/tutorials/Dietary/SurveyOrientation/Navigate
/Info4.htm)
How to Find the Analytic and Reporting Guidelines (/nchs/tutorials/Dietary/SurveyOrientation/Navigate/Frame4.htm)
Task 5: Find Other Useful Information Within the NHANES Website
Being familiar with other NHANES website resources will help you in preparing and conducting analyses. These resources
include the What's New Page, the Frequently Asked Questions Page, information about citing NHANES and referencing
NHANES' Institutional Review Board approval, and the email query system, and the NHANES listserv.
Key Concepts about Finding Other Useful Information Within the NHANES Website (/nchs/tutorials/Dietary
/SurveyOrientation/Navigate/Info5.htm)
How to Find Other Useful Information Within the NHANES Website (/nchs/tutorials/Dietary/SurveyOrientation/Navigate
/Frame5.htm)
Page last updated: May 3, 2013
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NHANES Dietary Web Tutorial - Dietary Data Overview https://www.cdc.gov/nchs/tutorials/dietary/SurveyOrientation/Navigate/i...
Page last reviewed: May 3, 2013
Content source: CDC/National Center for Health Statistics
Page maintained by: NCHS/NHANES
Centers for Disease Control and Prevention 1600 Clifton Road Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO
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NHANES Dietary Web Tutorial: Navigate the Website: Navigate the H... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 1: Key Concepts About Navigating the NHANES Homepage
The main section of the homepage serves as an outline of the entire NHANES site. At the top and the bottom of the homepage are various links to the
NHANES and National Center for Health Statistics (NCHS) websites. Some of these links and a few additional links are highlighted in the main
section. In the “How to” portion of this task, you will explore the various links on this page and gain an appreciation for the number and diversity of
materials available through the NHANES website.
This module does not provide an exhaustive explanation of all the web links. Instead, it will acquaint you with the website by providing an overall
description and showcasing a few selected links with relevant information important to analysts.
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NHANES Dietary Web Tutorial: Navigate NHANES Website: Task 1 https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 1: How to Navigate the NHANES Homepage
Step 1: Explore Links on the NHANES Homepage
Your first step in navigating the NHANES homepage is to find out what is contained in each link. The summary table below gives a brief description of
each link listed in the main section.
Links on NHANES Homepage and Descriptions
Link Description
About NHANES Provides a brief overview of NHANES and survey operations
What's New The most current information, including updates to existing data files and documents,
as well as new data releases.
Questionnaires, Data Sets Links to the current and historic NHANES datasets and documentation, the
and Related Documentation recommended NHANES citation, and FAQs.
Tutorials Links to a list of tutorials available from the NHANES website.
Proposal Guidelines Guidelines to follow to submit a proposal to use biological or DNA specimens, or
content for an upcoming cycle of the survey.
Survey Results and Survey Results and Products links to materials developed using NHANES data
Products published by NCHS staff, including the NHANES Selected Bibliography and Key
Statistics from NHANES. In addition, this section lists other related links, such as:
Healthy Eating Index
What We Eat in America
PubMed link specified to retrieve NHANES articles
Listserv Directions to subscribe to the NHANES listserv.
IMPORTANT NOTE
The NHANES website is constantly updated and modified as new information, data, and web designs are introduced. These updates are intended to
provide new information with better organization of the contents. However, it may also lead to certain disruptions or rearrangement to the site with
minimal notice. This tutorial uses the live NHANES website in modules dealing with navigating the website, or locating information on the web.
Therefore, tutorial users should be aware of possible website changes that may not be reflected immediately in the tutorial.
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Task 1: How to Navigate the NHANES Homepage
Step 1: Explore Links on the NHANES Homepage
Your first step in navigating the NHANES homepage is to find out what is contained in each link. The summary table below gives a brief description of
each link listed in the main section.
Links on NHANES Homepage and Descriptions
Link Description
About NHANES Provides a brief overview of NHANES and survey operations
What's New The most current information, including updates to existing data files and documents,
as well as new data releases.
Questionnaires, Data Sets Links to the current and historic NHANES datasets and documentation, the
and Related Documentation recommended NHANES citation, and FAQs.
Tutorials Links to a list of tutorials available from the NHANES website.
Proposal Guidelines Guidelines to follow to submit a proposal to use biological or DNA specimens, or
content for an upcoming cycle of the survey.
Survey Results and Survey Results and Products links to materials developed using NHANES data
Products published by NCHS staff, including the NHANES Selected Bibliography and Key
Statistics from NHANES. In addition, this section lists other related links, such as:
Healthy Eating Index
What We Eat in America
PubMed link specified to retrieve NHANES articles
Listserv Directions to subscribe to the NHANES listserv.
IMPORTANT NOTE
The NHANES website is constantly updated and modified as new information, data, and web designs are introduced. These updates are intended to
provide new information with better organization of the contents. However, it may also lead to certain disruptions or rearrangement to the site with
minimal notice. This tutorial uses the live NHANES website in modules dealing with navigating the website, or locating information on the web.
Therefore, tutorial users should be aware of possible website changes that may not be reflected immediately in the tutorial.
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NHANES Dietary Web Tutorial: Navigate the Website: Find Data Files https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 2: Key Concepts About Finding Datasets on the NHANES Website
Publicly-Released Datasets
Throughout the years, NHANES datasets and related information have been released in a variety of formats and different media. However, since the
late 1990s, all publicly available data and related documentation are released and updated in a centralized location: the NHANES website.
The website contains the public-use data files for each of the NCHS national surveys starting with the first National Health Examination Survey
(NHES I) dataset up to the most current dataset. Datasets contain data for people who participate in a selected survey. Currently, data files are
released in 2-year increments, or cycles (e.g., NHANES 2003-2004). The cycles since 1999 are referred to as “current NHANES” or “continuous
NHANES.” Codebooks and documentation are included with each of the data files. (For more information on locating data files and related
documentation, see the Locate Variables module and the Download Data Files module.)
Several pages, described in more detail in the How To section of this task, will be particularly important for preparing and conducting analyses:
Data Sets and Related Documentation,
Survey cycle, and
Data Documentation, Codebooks, SAS Code.
Data Sets and Related Documentation Page
The Data Sets and Related Documentation page lists all the survey cycles from the most recent to most historic. It also provides the NHANES
Analytic and Reporting Guidelines and the suggested citation for NHANES to use in publications.
Survey Cycle Page
The survey cycle page (titled by the survey cycle, e.g., NHANES 2003-2004) contains documentation about the survey, documentation on how to use
the data, and links to:
Contents at a Glance, which has information about the survey in general terms
Contents in Detail, which has detailed descriptions about the different items used in the survey including survey questionnaires, manuals,
brochures and consent forms
Using the Data, which has Analytic and Reporting Guidelines and documentation on how the data was released, and
Data, Documentation, Codebooks, SAS Code, which has links to the five component pages:
Demographics
Dietary
Examination
Laboratory
Questionnaire
Component Page
The component page (titled by the survey cycle and then the component name, e.g., 2001-2002 Demographics) links to the data file and
documentation for each component, as well as the variable list for each component. For Continuous NHANES (starting in 1999), the data files,
documentation, codebook and frequencies are available for every component. Starting with the 2003-2004 cycle, the codebook and frequencies were
included in the documentation file.
Non-Publicly Released Datasets
NHANES uses the following principles to guide the release of data. Data are to be released:
as widely as practicable,
as soon as possible after data collection, and
in as much detail as possible,
while maintaining survey participant confidentiality.
As a result, some variables or entire data files are not publicly released due to disclosure concerns, for example, geographic identifiers. These files
are only available through the Research Data Center (RDC). You may review the Data Release and Access Policy for more information.
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NHANES Dietary Web Tutorial: Navigate NHANES Website: Task 2 https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 2: How to Find Datasets
In this task, you will find where to:
1. download the publicly-released data files and documentation for the NHANES 2003-2004 Examination components, and
2. find information about accessing non-publicly released data.
Step 1: Download Publicly Released Datasets
From the homepage, shown here on the right, select the link titled Questionnaires, Data Sets and Related Documentation.
Then click the NHANES 2003-2004 link. Scroll down to the Data, Documentation, Codebooks, SAS Code section, and click the Examination link.
Review the variety of examination components available. This process can be repeated for reviewing the other links in the Data, Documentation,
Codebooks, SAS Code section.
Click your browser's Back button until you return to the NHANES homepage.
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Step 2: Find Information about Non-publicly Released Datasets
From the homepage, select the link titled Research Data Center (RDC) in the Related Sites box below the navigation panel.
Click the Step 1 - Types of Data button link.
Review the information in the Resctricted section to learn more about what types of restricted data are available through the RDC.
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NHANES Dietary Web Tutorial: Navigate the Website: Find Survey Bac... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 3: Key Concepts About Finding Survey Background Information
To determine the scope and details of your analysis, you may require background information about NHANES. Available background information
includes:
Survey Contents, which shows the years that data on different measures or components were collected and when changes in collection
methods occurred,
Sample Person Questionnaire, and
MEC Components Description, which is the survey protocol for obtaining the physical examination measures in the survey.
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NHANES Dietary Web Tutorial: Navigate NHANES Website: Task 3 https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 3: How to Find Survey Background Information
In this task, you will begin collecting background information about NHANES for your analysis. You will locate:
Survey Contents
Sample Person Questionnaire
MEC Components Description
Step 1: Find Survey Contents
From the homepage, shown here on the right, select the link titled Questionnaires, Data Sets and Related Documentation.
Next, select the NHANES 2003-2004 link.
Now, in the Contents at a Glance section, and click the Survey Contents link.
After the PDF file of the Survey Contents opens, go to the third page, where you will find a multi-page chart of NHANES components. A review of
this page will show you whether the components you are interested in were collected for all the years available or only some of the years. You can
then begin to design your analysis accordingly.
Step 2: Find Questionnaire Items
Click the Back button to return to the NHANES 2003-2004 page.
Select the Survey Questionnaires, Examination Components and Laboratory Components link in the Contents in Detail section.
If you were interested in dietary supplements data, you would scroll down to the Sample Person Questionnaire section and would select the Dietary
Supplements and Prescription Medications link. This link opens to the dietary supplement questions asked of the survey participants and lists
possible responses. If you wanted to analyze supplement use, you would want to review these questions, the possible answers, and whether or not
the question was part of a skip pattern.
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IMPORTANT NOTE
Remember, when you are designing your own analysis, the links you visit when finding questionnaire items will depend on your research question.
Step 3: Find MEC Component Descriptions
Click your browser's Back button to return to the Survey Questionnaires, Examination Components and Laboratory Components 2003-2004
page.
Scroll down to the Examination and Laboratory section and select the the Examination Components link.
A PDF file of the MEC Components Description opens. Review this file for descriptions of the components offered in the MEC. If you are interested in
linking your dietary analysis to an Examination component, such as a physical measurement, for example, review the Anthropometry portion, where
the protocol for taking those measurements is described.
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NHANES Dietary Web Tutorial: Navigate the Website: Find the Analytic... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 4: Key Concepts About Finding the Analytic and Reporting Guidelines
One of the most important documents to read before any analysis of NHANES data is the Analytic and Reporting Guidelines. These guidelines have
very useful and pertinent information about the survey and the data, including (but not limited to) summary recommendations about
file documentation resources,
complex survey design and weighting,
variance estimation procedures, and
sample size requirements.
This document presents analytic and reporting guidelines that should be used for NHANES data analyses and publications and provides a framework
to users for producing estimates that conform to the analytic design of the survey. The Analytic and Reporting Guidelines represent the latest
information from the National Center for Health Statistics (NCHS) on recommended approaches for analysis of all NHANES data, but with a particular
focus on data collected in the continuous NHANES (since 1999).
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NHANES Dietary Web Tutorial: Navigate NHANES Website: Task 4 https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 4: How to Find the Analytic and Reporting Guidelines
In this task, you will find the review the current version of the Analytic and Reporting Guidelines for information about analyzing NHANES data.
Step 1: Find Current Analytic and Reporting Guidelines
From the NHANES homepage, shown here on the right, select the Data Sets and Related Documentation link.
Next, select the NHANES Analytic Guidelines link. The first link listed on the page, The National Health and Nutrition Examination Survey
(NHANES) Analytic and Reporting Guidelines, is the most current version of the Guidelines.
Select The National Health and Nutrition Examination Survey (NHANES) Analytic and Reporting Guidelines link to open the PDF file of the
guidelines. Review the document and read the summary guidelines for information on:
complex survey design and weighting,
variance estimation procedures, and
sample size requirements.
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1 of 1 1/14/2019, 9:04 PM
NHANES Dietary Web Tutorial: Navigate the Website: Find Other Usefu... https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 5: Key Concepts About Finding Other Useful Information Within the NHANES Website
The following sections of the NHANES website provide other information that will be valuable to you as you prepare and conduct your analyses.
What's New page
The “What's New” page is an important resource for staying updated with current information about the survey. Currently, about 8-12 months of
updates and new releases are listed on this page. This page is updated continuously as new information becomes available. It's a good idea to
periodically visit this page to see whether your analysis will be affected because of new or updated data files.
Frequently Asked Questions
Frequently Asked Questions (FAQs) pertaining to each survey cycle are provided within each cycle's Data Sets and Related Documentation link.
These FAQ sections will help you with common questions about NHANES, such as:
When will more data be available from the survey?
Will data and weights be available on public use files for single years such as 1999, 2000, or 2001?
Why are there so many data files?
What is the sample size for a particular data item, questionnaire section, examination component, or laboratory analyte?
How are missing values, "blank but applicable," "don't know," and other values coded?
NHANES Institutional Review Board Approval
The NHANES website provides information that researchers need to document the NHANES Institutional Review Board approval in their proposals. It
also provides a suggested citation for NHANES data and supplies keywords for manuscripts using NHANES data that are submitted for
publication.
Help by Email
The NHANES website also provides two additional resources for researchers who need assistance. These are the NCHS email query system and
the NHANES listserv.
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NHANES Dietary Web Tutorial: Navigate NHANES Website: Task 5 https://www.cdc.gov/nchs/tutorials/Dietary/SurveyOrientation/Navigate/...
Task 5: How to Find Other Useful Information Within the NHANES Website
In this task, you will review other pages on the NHANES website to:
use the "What's New" page,
review frequently asked questions,
find important NHANES citations, and
get additional help.
Step 1: Use the What's New Page
On the NHANES homepage, shown here on the right, select the link titled What's New. You'll notice that the page is organized in descending order
by month and year, with information listed under one of three possible headings:
NEW, which lists new data items, including newly released data files,
UPDATED, which lists updated data items,
PUBLICATIONS, which lists recent data briefs, and
SPECIAL or ANNOUNCEMENTS, which includes information about events of particular note.
Select any link under these headings to go to the component page where you can download data files and related documentation. Use your browser's
Back button to return to the What's New page.
IMPORTANT NOTE
Links titled More may or may not be present on the What's New page. Selecting these links will tell you more about the changes that were recently
made to the updated file and in previous versions of the file, in reverse chronological order.
At the bottom of the What's New page, select the Archive - What's New link to review past listings on the What's New page.
Click the browser’s Back button twice to return to the NHANES homepage.
Step 2: Review Frequently Asked Questions (FAQs) Page
At the NHANES homepage, shown here on the right, select the Data Sets and Related Documentation link.
Next, select the NHANES 2003-2004 link.
Select the Frequently Asked Question link, under the Contents at a Glance heading.
Review the FAQs and select the link titled "Where can I find the analytic guidelines (weighting, variance estimation, sample design)?. The page
scrolls to the answer for the question. It says that the Analytic Guidelines contain information on the survey design and weighting information and
refers you directly to the analytic guidelines by way of an embedded link.
Review the list of questions for any other questions you may have and click them to read the answers.
Click the browser’s Back button twice to return to the NHANES homepage.
Step 3: Find Important NHANES Citations
From the NHANES homepage, shown here on the right, select the Introduction to NHANES and Mobile Examination Center link.
When the page opens, click the Institutional Review Board Approval link. A new page opens which shows the IRB approval and protocol number /
description. Review the contents of this page.
Click your browser's Back button twice to return to the NHANES homepage.
From the NHANES homepage, select the link titled Data Sets and Related Documentation. The Data Sets and Related Documentation page opens.
Click the Suggested NHANES Citation for Use in Publications link. The suggested citations for NHANES data, questionnaires, and examination
and laboratory protocol are detailed. In addition, keywords are suggested in identifying your work.
Click the browser’s Back button twice to return to the NHANES homepage.
Step 4: Get Additional Help
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National Center for Health Statistics (NCHS) Email Query System
To use the NCHS email query system, select the Contact Us link at the bottom or the top of any NCHS web page.
Select the Email NCHS Query link. A new email message will open and the To: field will be filled in.
IMPORTANT NOTE
If you do not want to use or do not have a default email client installed on your computer, you can copy the email address from the email message,
open your preferred email client, and paste the NCHS query email address in the To: field of a new message.
Individuals responding to your inquiry are knowledgeable about NHANES data. Every attempt will be made for a timely response (24-48 business
hours).
NHANES Listserv
Return to the NHANES homepage and click the animated graphic -- an envelope that switches to the text "Join the NHANES Listserv". Select the
body link. This opens your default email client and starts a message to [email protected].
IMPORTANT NOTE
If you do not wish to use your computer's default email client or do not have one set-up, open your preferred email software and enter
[email protected] in the To: field.
Leave the Subject: field blank.
In the message body type: subscribe NHANES Your Name, where Your Name is replaced with your first and last names.
Send the message. You have now subscribed to the NHANES listserv and will receive updates on NHANES activities and announcements of product
and data releases.
You should receive two initial emails from the listserv. One email details the output of the command you sent to the listserv and indicates if there were
any errors you need to correct. If the command is successful, you will receive a second email message from the listserv asking you to confirm your
request. Reply to the message and type ok in the message body.
If your command confirmation is successful, you will receive three email messages:
one with the output of your confirm command,
one stating that you are now subscribed to the NHANES listserv and details about how to use the listserv, and
one with the usage guidelines that outline the purpose of the listserv and what activities are unacceptable.
If your command confirmation is not successful, then read the additional instructions in the command confirmation email and, following the
suggested changes, try to confirm the command again. If you need additional help, click the Listserv Help link at the bottom of the page. This page is
the Help page for all the National Center for Health Statistics' listservs and has troubleshooting tips for subscribing and unsubscribing.
FSRG Listserv
The FSRG Listserv, maintained by USDA, provides email notices to alert subscribers about new data releases and products for What We Eat in
America, NHANES. The FSRG Listserv does not allow interactive discussion. Click on the following link http://www.ba.ars.usda.gov/foodsurvey
/listserv.html and provide your name and email address to join.
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Dietary Data Structure & Contents
Purpose
This module describes how Continuous NHANES dietary data are structured and organized. In this context, structure refers to
the way the data are organized within the NHANES framework.
Task 1: Describe How Dietary Data Fit into the NHANES Data Structure
Before you begin a search for any specific diet-related variables on the website, you should first understand the NHANES data
structure and how diet files fit in.
Key Concepts about How Dietary Data Fit into the NHANES Data Structure (/nchs/tutorials/Dietary/SurveyOrientation
/DietaryDataStructureContents/Info1.htm)
Task 2: Describe Dietary Recall Data Files
Data collected from the dietary recall interviews are found in four files located in the Examination section.
Key Concepts about Dietary Recall Data Files (/nchs/tutorials/Dietary/SurveyOrientation/DietaryDataStructureContents
/Info2.htm)
Task 3: Describe Food Frequency Data Files
Data collected from the food frequency questionnaires are found in two files located in the Questionnaire section.
Key Concepts about Food Frequency Data Files (/nchs/tutorials/Dietary/SurveyOrientation/DietaryDataStructureContents
/Info3.htm)
Task 4: Describe Dietary Supplement Data Files
Data collected from the dietary supplement interviews are found in five files located in the Questionnaire section.
Key Concepts about Dietary Supplement Data Files (/nchs/tutorials/Dietary/SurveyOrientation/DietaryDataStructureContents
/Info4.htm)
Page last updated: May 3, 2013
Page last reviewed: May 3, 2013
Content source: CDC/National Center for Health Statistics
Page maintained by: NCHS/NHANES
Centers for Disease Control and Prevention 1600 Clifton Road Atlanta, GA 30329-4027, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO
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Task 1: Key Concepts About How Dietary Data Fit Into the NHANES Data Structure
Cycles
The Continuous NHANES data are released in two-year cycles beginning with NHANES 1999-2000 (See the "Overview of NHANES Survey
Design and Weights" module for information about the reasons for this policy.) Some survey topics areas are included in all survey cycles, whereas
others are included in selected cycles.
Sections
The data for each survey cycle are composed of five sections: Demographics, Dietary, Examination, Laboratory, and Questionnaire.
Demographics files contain survey design variables such as weights, strata and primary sampling units, as well as demographic
characteristics, such as sex, age, and race.
Dietary files contain information collected on the diet of participants. These files include data from the dietary recall data, food frequency and
dietary supplements questionnaires, as well as a number of diet related questions that follow the dietary recall in the Mobile Examination
Center.
Examination files contain information collected through physical exams (including anthropometric and physical activity data), and dental
exams.
Laboratory files contain results from analyses of blood, urine, hair, tuberculin skin test, air, and household dust and water specimens. These
files also include serum nutrient and cholesterol values, which can be used for nutritional assessment.
Questionnaire files contain data collected through household and MEC interviews and mailed questionnaires.
Each section contains groups of related variables, packaged in one or more data files for each 2-year cycle. This organization scheme allows for
efficiency in posting data to the website, downloading them, and analyzing them.
For each cycle of the continuous NHANES, the 1999-2010 Survey Content Brochure provides:
data files associated with each section,
age ranges for which data on specific variables were collected, and
notes describing any additional (non-age-based) exclusions.
The following table lists examples of the diet-related survey data files and topics that were included in NHANES 2003-2004.
Diet-Related Survey Components in NHANES 2003-2004
Section Data Topics Data Collection Setting
Examination
Dietary recalls (4 data files) Mobile Examination Center (MEC)
Laboratory Body composition (first dietary recall)
Questionnaire Body measurements Telephone follow-up (second dietary
Blood pressure examination recall)
Physical activity monitor
Cardiovascular fitness
Serum cholesterol and MEC
lipoproteins
Serum and red blood cell folate
Iron status and hematology
Other nutritional biochemistry
data
Blood pressure Household (HH) interview
Cardiovascular disease
Current health status (self-
reported)
Diabetes
Dietary supplements (5 data
files)
Medical conditions
School meal program
participation
Food stamp program
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Section Data Topics Data Collection Setting
participation MEC dietary interview (Day 1)
Dietary interviews (Days 1 and 2)
Fish & shellfish consumption MEC private interview
Table salt use
HH and MEC private interviews
Plain water intake Post-exam mailed questionnaire
HH and post-MEC phone interviews
Alcohol use HH and MEC interviews
Reproductive health (including
breast feeding)
Self-reported pregnancy status
Weight history
WIC participation
Food Frequency Questionnaire
(2 data files)
Special diets
Food security
IMPORTANT NOTE
This NHANES diet tutorial focuses primarily on dietary recall data, dietary supplement data, and food frequency data. However, the principles
described and analyses demonstrated in the tutorial also apply to the other types of dietary data collected in NHANES.
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Task 2: Key Concepts About Dietary Recall Data Files
Beginning in 2002, two 24-hour recalls were requested from all NHANES examinees. All Day 1 interviews were conducted at the Mobile Examination
Center using an in-person mode of interview. The Day 2 recalls were collected approximately three to ten days after the examination using a
telephone mode of interview.
The main data are captured in four files, found on the 2003-2004 Dietary Files page. (Note that additional data, derived from these main files, can be
found on the USDA Web site. See “Module 4: Resources for Dietary Data Analysis” for more information.)
NHANES 24-Hour Recall Interview Data Files
Data File File Name Data Contained in the Data File
Dietary Interview (Individual DR1IFF_C Detailed description about foods reported (i.e.
Foods – First Day) type, form, brand name, amount consumed)
Amounts of nutrients from each food
Dietary Interview (Individual DR2IFF_C Detailed description about foods reported (i.e.
Foods – Second Day) type, form, brand name, amount consumed)
Amounts of nutrients from each food
Total Nutrient Intakes (First DR1TOT_C Total energy and nutrient intakes from foods on
Day) recall day
Total amount of water consumed on recall day
Recall day's consumption amount compared to
typical diet
Information on added salt (frequency and type)
Whether on a special diet and type of diet
Frequency of fish and shellfish consumed (past 30
days)
Total Nutrient Intakes DR2TOT_C Daily total energy and nutrient intakes from foods
(Second Day) Daily amount of water consumed on recall day
Recall day's consumption amount compared to
typical diet
Be aware that the first two files, relating to individual foods, contain multiple records per person. The second two files, relating to total nutrient
intake, contain a single record per person. This difference is illustrated below. These files can be linked by sequence number (SEQN), the variable
that identifies individual NHANES participants, and the observations are indexed by line number.
Also be aware that the individual foods files contains records only for participants with complete intakes that were considered to be reliable
(DR1DRSTZ=1), with the exception of those individuals who reported no food on a day even though their intakes were considered complete because
they were fasting. The total nutrient intakes files, on the other hand, have records for all persons with complete intakes, including the fasters, but also
have records for those with incomplete/unreliable intakes and those who did not participate in the dietary recall interview at all. Therefore, individuals
with incomplete/unreliable intakes have no records in the individual foods file; however, they do have a record in the associated total nutrient file but
the total nutrient variables contain missing values.
Example from Individual Foods Files of Multiple Food Records Per Person
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Notice that in this illustration of a partial food record printout, the participant associated with SEQN 27530 reported consuming 2 foods (denoted by
the food codes), which gives this participant two records for each variable in the dietary recall file (e.g., the calories from food and calcium from food
shown here). The participant associated with SEQN 30888 reported consuming 25 foods, which gives this participant 25 records for each variable in
the dietary recall file.
Example from Total Nutrient Intakes Files of One Record Per Person
Notice that in this illustration of a partial nutrient record printout, each SEQN is associated with one record for each of the variables in the table. That
is because each record represents a total derived from summing individual records. This is in contrast to the previous table in which SEQN was
associated with more than one record per variable, depending on the number of foods that the participant reported consuming.
IMPORTANT NOTE
The Dietary Files page has links to each of these data files as well as to the documentation and the procedures used to collect the data.
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Task 3: Key Concepts About Food Frequency Data Files
Food Frequency Questionnaire
A Food Frequency Questionnaire (FFQ) was used in NHANES 2003-2004 and NHANES 2005-2006 to collect information on the frequency with which
selected food items were consumed during the past 12 months. Along with the two 24-hour dietary recall interviews and interview information on
dietary supplement use, food security, and dietary behavior, the FFQ completes the dietary assessment portions of the survey. The FFQ was mailed
to all participants aged 2 and older who completed at least one 24-hour recall. The purpose of the FFQ is to be used in conjunction with the 24-
hour recall data to model usual intake.
The NHANES Food Frequency Questionnaire asks participants about their consumption of more than 100 different food items. Specifically, there are
151 frequency questions, which include follow-up questions querying intake of certain foods over two seasons (i.e. summer OR winter versus rest of
year). The FFQ also includes follow-up questions asking about the proportion of the time certain subtypes of the food were eaten over the past 12
months. In contrast to some other food frequency questionnaires, the NHANES Food Frequency Questionnaire does not query portion size.
A sample FFQ question for soft drink consumption is listed below. Notice that the question includes an introductory “stem” question to ascertain if the
food was consumed in the past 12 months. If the participant answered “No” they are directed to the next question. If the participant answers
affirmatively, the stem branches to additional questions about seasonal soda consumption and types of soda (e.g., diet soda and caffeine-free soda).
The resulting data files provide frequency information for all these questions which can be collapsed to examine intake for all soda combined, or by
type.
Example of Food Frequency Questionnaire
Food Frequency Questionnaire Data Files 1/14/2019, 9:15 PM
These data are captured in four files, found on the 2003-2004 Dietary Files page.
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Food Frequency Questionnaire Data Files
Data File File Name Data Contained in the Data File
FFQ FFQRAW_C Information on “raw” responses (without algorithms
Questionnaire applied) to every question on the FFQ; Frequencies from
this file are converted, using algorithms, to daily
File frequencies that are found in the FFQ Daily Frequency
Covariates File
FFQ Daily Variables include SEQN, FFQ_FOOD, FFQ_VAR, and
Frequency FFQDC_C FFQ_FREQ
Covariates File FFQ_FREQ represents contributions to the average daily
frequency of intake of all foods queried on the FFQ. For
foods that are queried differentially by season or for foods
with multiple subtypes, data represent separate
contributions to the yearly total.
FFQ_FOOD indexes subtypes of food (e.g. skim, whole,
2% milk)
FFQ_VAR indexes foods consumed by season
FFQ Variable VARLOOK Descriptions corresponding to FFQ_VAR codes
Look-up File Descriptions corresponding to FFQ_FOOD codes
FFQ Food Look- FOODLOOK
up File
The FFQ Questionnaire File, which provides the reported “raw” responses to every question on the FFQ, contains one record per person. Each
record contains data for all questions on the FFQ, and data are coded using a separate variable for each question. The variable naming convention is
FFQXXXXX, where XXXXX equals the question number (e.g., FFQ0009C corresponds to Question 9c, which is “How often were these soft drinks,
soda, or pop diet or sugar-free?”). Data in the Questionnaire File are not edited. The FFQ Questionnaire file is rarely used; rather, analysts of FFQ
data almost always use values from the FFQ Daily Frequency Covariates File. On the rare occasion that one might wish to combine data from these
files, they can be linked by the individual ID number, SEQN.
The FFQ Daily Frequency Covariates File contains multiple records per person; there is a separate record for each food subtype/season
combination. NCI’s DietCalc software applies algorithms to the raw frequency responses from the FFQ Questionnaire File to convert them into
average daily frequencies over the past year (for more details on these algorithms, please see the FFQ documentation on the NHANES Web site).
The DietCalc software also imputes data in cases where there are inconsistent results for the stem and follow-up questions (e.g. when a participant
reports that they never ate a certain food, but provided answers to the follow-up questions).
Average daily frequencies are indexed by two variables; these two variables are:
FFQ_VAR: indexes food by season (i.e. in summer/winter OR “in season” versus rest of the year).
FFQ_FOOD: indexes subtypes of food, such as different types of beverages.
The FFQ Daily Frequency Covariates file contains a separate record for each subtype and season combination of each food asked on the FFQ. Each
record designates the season (using FFQ_VAR), the subtype (using FFQ_FOOD), and the contribution of that subtype/season combination to
the daily frequency over the year(using FFQ_FREQ). For foods that are not queried by season, FFQ_FREQ represents the average daily
frequency of consumption of that food. When foods are queried by season and/or subtype, frequencies are presented in such a way that they can be
summed to get the average frequency of consumption. These frequencies were weighted using algorithms to take into account nonequivalent
lengths of time (e.g. “summer” is considered to be 4 months whereas “rest of year” is 8 months) so that summing the frequencies provides the overall
daily frequency over the past year. Therefore, frequencies for a food or beverage consumed in a particular season do not accurately represent daily
frequencies for that season. Instead, when foods are queried by season, it is necessary to sum by FFQ_FOOD (i.e. foods with the same value for
FFQ_FOOD) to capture the contributions of the different seasons.
The last two files listed above, FFQ Variable Look-up File (VARLOOK) and FFQ Food Look-up File (FOODLOOK), are used to cross-reference
numerical codes in the FFQ Daily Frequency Covariates File to corresponding food descriptions. VARLOOK provides descriptions for FFQ_VAR
codes and FOODLOOK provides descriptions for FFQ_FOOD codes. These files can be merged with the FFQ Daily Frequency Covariates File using
the sample SAS program (FFQmerge.SAS) available on the Downloads page.
A complicated example of using the FFQ data occurs with soda/soft drinks, for which participants are asked to report intake by season, and by
subtypes of regular/diet or with/without caffeine. As shown below in the snippet of output for Participant Number 21007, in order to determine the
average daily frequency of intake of diet soft drinks, you would need to sum all of the FFQ_FREQ values designated by FFQ_FOOD values of 182
or 183 (shown in yellow). In this case, you would be summing by values of FFQ_FOOD irrespective of what time of year the diet soft drinks were
consumed. Thus, participant 21007 consumed diet soft drinks an average of 0.35 times per day, or about once every three days. Similarly, if you
were to add the FFQ_FREQ values designated by FFQ_FOOD values of 180 or 181 (shaded in orange), you would get the average daily frequency
of regular soft drink consumption. Participant 21007 consumed regular soft drinks an average of 0.35 times per day, as well.
It would also be possible to determine how many times per day this participant consumed regular caffeinated soft drinks or regular decaffeinated soft
drinks by summing rows where FFQ_FOOD is 180 or 181, respectively. Furthermore, you could determine how many times per day participant 21007
consumed all types of caffeinated soda or all types of decaffeinated soda by summing rows where FFQ_FOOD is 180 & 182 OR 181&183,
respectively.
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FFQ_FREQ Output for Respondent Sequence Number 21007
FFQ_FREQ FFQ_VAR VAR_VALUE FFQ_FOOD FOOD_VALUE
0.000 1 Tomato Juice, Veg Juice 130 Tomato/veg juice/all
0.79 2 Orange juice or grapefruit juice 128 Orange/grpfrt jce/all
0.21 3 Other fruit juice 129 Other juice
0.08 4 Fruit drinks: Hi-C, lemonade 184 Fruit drinks/ reg
0.50 5 Milk (as a beverage) 12 Milk/ whole to drink
0.00 6 Meal Replacement, Energy/High-Protein Beverag 25 Meal repl/ liquid
0.06 7 Soda in Summer 180 Soft drinks/ reg/ caff
0.02 7 Soda in Summer 181 Soft drinks/ reg/ decaf
0.06 7 Soda in Summer 182 Soft drinks/ diet/ caff
0.02 7 Soda in Summer 183 Soft drinks/ diet/ decaf
0.20 8 Soda, Rest of Year 180 Soft drinks/ reg/ caff
0.07 8 Soda, Rest of Year 181 Soft drinks/ reg/ decaf
0.20 8 Soda, Rest of Year 182 Soft drinks/ diet/ caff
0.07 8 Soda, Rest of Year 183 Soft drinks/ diet/ decaf
…Continued…
IMPORTANT NOTE
The Dietary Files page has links to each of these data files as well as the documentation and codebooks.
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Task 4: Key Concepts About Dietary Supplement Files
Information on use of vitamin, mineral, herbal, and other dietary supplements is collected from all NHANES participants during the household
interview.
These data are captured in five files, found on the 2003-2004 Dietary Files page.
Dietary Supplement Data Files
Data File File Data Contained in the Data File
Supplement Counts Name
participant ID number (SEQN number)
a variable to indicate whether or not a dietary
supplement was taken
DSQ1 total number of supplements taken by the
participant in the past 30 days
whether or not a non-prescription antacid had been
taken and how many
Participant's Use of participant ID number (SEQN number)
Supplement (separate records DSQ2 supplement name and ID number
for each supplement taken) whether the supplement container was seen
how long the person has been taking this
supplement
how often supplement taken in the past 30 days
how much supplement was taken each time
Dietary Supplement Product DSPI supplement name and ID number
Information general information about the supplement from the
NCHS database derived from the supplement label
source of the label information
type of supplement formulation
the number of vitamins, minerals, amino acids,
botanicals, and other ingredients listed on the
supplement label
the serving size upon which the amounts in the
Supplement or Nutrition Facts box are based
form of the serving size (e.g., tablet, softgel, spray,
teaspoon, ml)
Dietary Supplement Ingredient DSII supplement name and ID number
Information ingredient ID (ingredients that are in each
supplement reported by a participant for which the
amount of the ingredient is known, including blends)
Dietary Supplement Blend DSBI ingredient ID
Information ID numbers and names of proprietary blends that
are also listed as ingredients in File 4
components of the blend that have no amounts, are
included in this file.
IMPORTANT NOTE
The Dietary Files page has links to each of these data files as well as to the documentation and the procedures used to collect the data.
Be aware that File 1 contains one record per person. File 2 contains a variable number of records per person, from 0 to multiple records, depending
on whether and how many supplements the participant consumed. Files 3, 4, and 5 contain information only about the supplements and their
ingredients. As shown in the following diagram, the files of this hierarchical data structure can be linked by participant ID, supplement ID, or
ingredient ID.
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Data File Structure and Relationships
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Resources for Dietary Data Analysis
Purpose
Researchers often employ sources of information other than the data themselves when conducting dietary analysis studies.
These include databases, search tools, and food grouping schemes. Most of these research resources are used with the dietary
recall data but some also can be used with either the dietary supplement or food frequency data.
Task 1: Describe the Food and Nutrient Database for Dietary Studies (FNDDS)
The Food and Nutrient Database for Dietary Studies, or FNDDS, is a database of foods, their nutrient values, and their gram
weights for typical food portions. It is used to process the NHANES dietary recall data that are included in the two types of
dietary data files (individual foods file and total nutrients file) and also can be used as a resource in analyzing NHANES dietary
data.
Key Concepts about the Food and Nutrient Database for Dietary Studies (/nchs/tutorials/Dietary/SurveyOrientation
/ResourceDietaryAnalysis/Info1.htm)
How to Access the Food and Nutrient Database for Dietary Studies (/nchs/tutorials/Dietary/SurveyOrientation
/ResourceDietaryAnalysis/frame1.htm)
Task 2: Describe the USDA Food Coding Scheme
The USDA food coding scheme provides a numeric identifier based on hierarchical grouping for each of the thousands of foods
in the FNDDS. The coding scheme may be used to categorize individual foods into groups that are appropriate for your
research needs. This is important because similar foods are often grouped, rather than analyzed individually, in research
studies. You can use this coding scheme to develop your own food groups. Alternatively, you can use the food groups defined
by USDA (see Task 3).
Key Concepts about the USDA Food Coding Scheme (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis
/Info2.htm)
How to Access the USDA Food Coding Scheme (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis/frame2.htm)
Task 3: Describe Food Surveys Research Group (FSRG)-Defined Food Groups
USDA's Food Surveys Research Group (FSRG) has developed a grouping scheme to aid in reporting food intake estimates. It
groups the foods in the FNDDS into 71 food groups and subgroups, and the documentation contains a short description of the
foods included and excluded in each.
Key Concepts about the FSRG-Defined Food Groups (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis
/Info3.htm)
How to Access the FSRG-Defined Food Groups (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis
/frame3.htm)
Task 4: Describe the What's In the Foods You Eat Search Tool
The USDA's What's in the Foods You Eat tool is a database that allows you to search for foods included in the FNDDS. This
search tool is useful for accessing information about individual as well as grouped foods.
Key Concepts about the What's in the Foods You Eat Search Tool (/nchs/tutorials/Dietary/SurveyOrientation
/ResourceDietaryAnalysis/Info4.htm)
How to Use the What's In the Foods You Eat Search Tool (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis
/frame4.htm)
Task 5: Describe the MyPyramid Equivalents Database
The MyPyramid Equivalents Database (MPED) provides MyPyramid food group amounts, expressed as numbers of cup
equivalents, ounce equivalents, teaspoon equivalents, or grams, in the foods reported eaten in a recall day. All foods in the
FNDDS identified by food codes (e.g., lasagna) are disaggregated into their component ingredients (e.g., pasta, mozzarella,
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beef), and these ingredients have been converted to equivalent amounts of relevant MyPyramid food groups (e.g., grains, milk,
meat).
Key Concepts about the MyPyramid Equivalents Database (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis
/Info5.htm)
How to Access the MyPyramid Equivalents Database (/nchs/tutorials/Dietary/SurveyOrientation/ResourceDietaryAnalysis
/frame5.htm)
Page last updated: May 3, 2013
Page last reviewed: May 3, 2013
Content source: CDC/National Center for Health Statistics
Page maintained by: NCHS/NHANES
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Task 1: Key Concepts About the Food and Nutrient Database for Dietary Studies (FNDDS)
The FNDDS is a database of foods, their nutrient values, and their weights for typical food portions. It consists of three components, including the
food descriptions component, the food portions and weights component, and the nutrients component. These three components are used to process
and analyze NHANES dietary recall data.
Food Description Component
The Food Descriptions Component contains descriptions for about 13,000 foods. Every food in the database has an 8-digit code associated with it.
These codes are the link between the FNDDS and the recall data. These codes are used to identify foods in each of the three FNDDS components.
Each food description has two versions—a complete, 200 character version and an abbreviated, 60-character version, which is written in capital
letters. Here is an example of a food description for food code 27345410.
Example Food Description - 27345410
Complete Chicken or turkey, rice, and vegetables (including carrots, broccoli, and/or
description: dark- green leafy), cream sauce, white sauce, or mushroom soup-based
sauce (mixture)
Abbreviated
description CHIX, RICE, & VEG (INCL CAR/DK GRN),CR/SOUP-BASED SAU
When dietary recall data are coded and processed, the Food Descriptions Component is used to convert the foods reported by participants to the
appropriate USDA food code.
Food Portions and Weights Component
The Food Portions and Weights Component contains the weights, in grams, for common portions of each food in the FNDDS. The FNDDS contains
about 30,000 food portion weights. When dietary intake data are coded and processed, this component is used to convert the amount of each food
reported by the participant into gram weights. Only the gram weights are included in the Individual Foods File. The gram weights are used, along
with the FNDDS nutrient values, to calculate the nutrient content of each food amount.
Nutrients Component
The Nutrients Component includes values for the amount of food energy and nutrients/food components contained in 100 grams of each food in the
database. The FNDDS nutrient values are used to calculate the nutrient amounts provided by each food reported on the dietary recalls.
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Task 1: How to Access the Food and Nutrient Database for Dietary Studies (FNDDS)
Step 1: Locate FNDDS
Go to the FSRG's website at http://www.ars.usda.gov/ba/fsrg.
From the home page, select the link titled Food and Nutrient Database for Dietary Studies. This link will take you to the home page for the
FNDDS.
Step 2: Locate FNDDS overview
Click the What is FNDDS and How Can You Use It link for a brief overview and applications of the database, currently available versions, and their
citations.
Click your browser's Back button to return to the FNDDS home page.
Step 3: Select version of FNDDS
The FNDDS is updated for each 2-year release of the dietary recall (What We Eat In America, or WWEIA) data. FNDDS version 1.0 corresponds with
NHANES 2001-2002, and FNDDS version 2.0 corresponds with NHANES 2003-2004. Updated versions of the FNDDS incorporate the ever-
expanding list of food codes used in the surveys.
For each FNDDS version, the FNDDS home page provides the following links:
Documentation—takes you to a PDF document that includes an extensive discussion of the database. Links for information about changes
that have been made to the database (Changes From Previous Version) and Data Files and Formats are also provided on the home page.
List of Nutrients/Food Components – includes the nutrients and food components that are in the database for a given cycle.
Download database – includes file format options, instructions, and links for downloading each database.
Suggested Citation
What's In The Foods You Eat Search Tool—is specific to each FNDDS version and includes the food descriptions, food portions and
weights, and nutrient values included in the particular database.
Food Coding Scheme – provides an overview of how FNDDS foods are classified and provides links to view the coding scheme for FNDDS
2.0 and FNDDS 1.0.
Frequently Asked Questions
References and Related Links
Step 4: Access Vitamin A and E databases
The FNDDS home page provides a link to the USDA Database of Vitamin A (mcg RAE) and Vitamin E (mg AT) for NHANES 1999-2000.
Beginning with FNDDS 1.0, values for vitamin A and vitamin E are expressed as micrograms of retinol activity equivalents (mcg RAE) and milligrams
of alpha-tocopherol (mg AT), respectively. However, food intake values for the original release of NHANES 1999-2000 were expressed as
micrograms of retinol equivalents (mcg RE) for vitamin A and as milligrams of alpha tocopherol equivalents (mg ATE) for vitamin E.
Using this database, vitamins A and E intake estimates for the NHANES 1999-2000 survey can be calculated according to the units currently in use.
For more information about this conversion, visit the Documentation, Download Database, and Suggested Citation links that are provided for this
database.
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Task 2: Key Concepts About the USDA Food Coding Scheme
The USDA food coding scheme is a way of organizing the thousands of food codes used in NHANES and can be used as a guide to grouping foods
for your particular research needs.
Individual Food Codes
The first digit of each USDA food code represents one of nine major commodity groups listed below.
Table of USDA Food Code First Digits and Major Commodity Groups
Identifier Description
1 Milk and milk products
2 Meat, poultry, fish, and mixtures
3 Eggs
4 Legumes, nuts, and seeds
5 Grain products
6 Fruits
7 Vegetables
8 Fats, oils, and salad dressings
9 Sugars, sweets, and beverages
The second, third, and sometimes fourth digits of a code identify increasingly more specific subgroups within the nine major groups. The
remaining digits are used to identify specific foods within a numerical sequence. Here is an example for milk and milk products.
Example of Food Code Sequence and Relationship to specific food subgroups
Food Code Digits Group/subgroup/description
1- Milk and milk products
11- Milk and milk drinks
115- Flavored milk and milk drinks
11511100 Milk, chocolate, whole-milk-based
It is important to note that some individual food codes represent discrete food items while others represent mixed dishes consisting of multiple
ingredients. If a food code represents a mixed dish, it is grouped according to its major ingredient. See Mixed Dishes, below, for more information on
this topic.
Combination Codes
During the collection and coding of dietary recall data, many individual food codes are linked together using “combination” codes. These
codes allow investigators to account for individual foods that are consumed simultaneously, such as sugar in coffee or milk on cereal, or for food
mixtures that are reported as discrete ingredients, such as a homemade sandwich reported separately as bread, cheese, lettuce and mayonnaise.
Combinations are defined using two separate variables. The first, the combination food number, flags foods as being eaten in combination. Each
combination is given a unique combination food number, and these are listed in sequence (i.e., the first food combination reported by a participant is
1, the second is 2, and so on). The second variable, the combination food type, designates the type of combination, as shown below in the list.
Code Combination Food Types
00
01 Description
Non-combination
Beverage with additions
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02 Cereal with additions
03 Bread/baked products with additions
04 Salad
05 Sandwiches
06 Soup
07 Frozen meals
08 Ice cream/frozen yogurt with additions
09 Dried beans and vegetable with additions
10 Fruit with additions
11 Tortilla products
12 Meat, poultry, fish
13 Lunchables
90 Other mixtures
The following example shows the relationship of the combination food number and combination food type for three food items. Note that all the foods
in a given combination are assigned the same combination food type code and combination food number.
Combination Foods Example
Combinations Food Items Combination food Combination food
eaten reported number type
Cereal with fruit and Frosted flakes cereal 01 02
milk Milk 01 02
Bananas 01 02
Coffee with sugar
Coffee 02 01
Ham and cheese Sugar 02 01
sandwich
Ham 03 05
Cheese 03 05
Bread 03 05
Mustard 03 05
Pickle 03 05
Foods with a value of 00 for both combination number and combination type are either discrete food items that were not eaten in combination or
mixed dishes coded with a single food code.
Mixed Dishes Code
Mixed dishes include food items such as stews, soups, casseroles, sandwiches, pasta with meats and sauces, pizzas, and tortilla dishes (such as
enchiladas and burritos). As mentioned above, mixed dishes can be coded either by an individual food code or several food codes—
representing the ingredients of the mixture—linked by a combination food number and combination food type.
Individual food codes representing mixed dishes are included in many food groups and subgroups of the coding scheme. Generally, mixtures
represented by individual food codes are placed in food groups based on the primary component or ingredient in the mixture. For example, a
cheeseburger on a bun is assigned to the “meat, poultry, fish and mixtures” group because the hamburger is considered to be the main ingredient.
Lasagna with meat is assigned to the “grain products” group because the noodles are considered to be the main ingredient.
Certain types of mixtures, such as sandwiches, salads, and soups, can be included in various food groups, depending on their main ingredient.
Therefore, if you are interested in a particular type of mixture, it is important to note all the possible food groups that could contain codes related to
that mixture. For example, different kinds of sandwiches can be found in various food groups and subgroups, as shown below.
Examples of Mixed Dishes Food Codes
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Groups of Codes
Because of the level of detail in the descriptions for many FNDDS foods, it is unlikely that you will examine recall data by individual food codes. It is
more likely that you will combine food codes representing similar foods into food groups and examine the data by food group.
If you plan to develop a food grouping scheme for your study, your analysis should take into consideration the widespread use of foods coded as
multiple items and linked by a combination food number and type. In NHANES 2003-2004, about one-half of all food items were recorded as
combination foods.
It may be useful to conduct a preliminary review of the data to ensure that your groups are going to be meaningful. Look at how often foods in your
group(s) are reported and also the gram amounts consumed. Then decide whether to combine groups or to disaggregate to a greater level of detail.
USDA's Food Surveys Research Group has developed a set of food groups for their reports and analyses. These food groups are currently based on
the 8-digit food code. To learn more about these, see the “Task 3: How to Access the FSRG-Defined Food Groups".
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Task 2: How to Access the USDA Food Coding Scheme
Step 1: Locate FNDDS homepage
Go to the FSRG's website at http://www.ars.usda.gov/ba/fsrg.
Select the linked titled Food and Nutrient Database for Dietary Studies. This link will take you to the home page for the FNDDS.
Step 2: Locate USDA Food Coding Scheme
Scroll down to and select the Food Coding Scheme link. This link opens a page which provides an overview of the food coding scheme and
includes links for the FNDDS 2.0 and FNDDS 1.0 coding schemes.
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Task 3: Key Concepts About the Food Surveys Research Group (FSRG)-Defined Food
Groups
Food groups were developed by the Food Surveys Research Group (FSRG) for use in reporting estimates of food intakes in past USDA food
surveys. In this scheme, foods are classified into 71 food groups and subgroups. For each food group or subgroup, a short description of the kinds of
food that are included and excluded is provided and the food codes included in each group/subgroup are identified. You can use these food groups in
your analysis or you can devise your own grouping scheme.
As an example, here is the description for the dried fruit subgroup. The list of food codes for this group is very simple, with all codes beginning in
621-. Some other groups have much more complicated lists.
Dried fruits
Includes dried apples, apricots, dates, prunes, raisins, and other dried fruits
Excludes juices such as prune juice, which are included in “other fruits, mixtures, and juices”
Food codes: 621- (i.e., all codes that begin with 621)
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Task 3: How to Access the Food Surveys Research Group (FSRG)-Defined Food Groups
Step 1: Locate FNDDS documentation
Go to the FSRG's website at http://www.ars.usda.gov/ba/fsrg.
Select the linked titled Food and Nutrient Database for Dietary Studies. This link will take you to the home page for the FNDDS.
Scroll down and select the Documentation link listed under FNDDS 2.0.
IMPORTANT NOTE
Note that the FSRG-Defined Food Groups differ slightly for each NHANES survey cycle, so it is important to choose the correct documentation link.
Step 2: Locate FSRG-Defined Food Groups in FNDDS documentation
The Documentation link opens to the first page of the documentation. Bookmark tabs are listed on the left side. Click on the tab entitled
“Appendixes” and scroll through the document until you come to “Appendix C. FSRG-Defined Food Groups”
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Task 4: Key Concepts About the What's In the Foods You Eat Search Tool
The USDA What's in the Foods You Eat is a tool allows you to easily search the FNDDS. This search tool is available for each FNDDS version. The
online version of the search tool corresponds to the most recently released version of the FNDDS. The current and previous versions of the search
tool can be downloaded from the FSRG website.
In addition to accessing information about individual foods, this search tool can be useful when grouping foods. Specifically, the search tool can be
used to see descriptions of specific foods, to get an idea of how many food codes are available for a prospective group, or to scroll through all the
foods in a defined range of numeric food codes.
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Task 4: How to Use the What's In the Foods You Eat Search Tool
You can use the What's in the Foods You Eat search tool to search for foods by food name or keyword, compare nutrients between foods, and search
for foods by food code. You can also download the search tool to your computer by following the instructions on the Food Surveys Research Group
(FSRG) website. This will allow you to access the search tool even when you are not connected to the Internet.
Step 1: Search by food name or keyword
Go to the USDA What's in the Foods You Eat home page at http://www.ars.usda.gov/foodsearch.
Select the Click Here link used to access the Online tool.
In the Keywords/food code box, enter the name of a food. We will use “milk” as an example. Click the Search button or hit the Enter key.
Notice that 351 food codes were found. Remember that this value is based on FNDDS 2.0. Future versions may yield slightly different results.
Also notice the different terms used in the “Description” column to distinguish between the food codes and the inclusion criteria listed in the “Includes”
column.
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Step 2: Compare nutrients between foods with differing descriptions
To compare nutrients between foods with differing descriptions, use the following procedures. In this example, we will use the search tool to
determine the difference in calcium content between one cup of "Milk, cow's, fluid, whole" and one cup of "Milk, calcium fortified, cow's, fluid, whole."
Click on the food code number 11111000 with the description "Milk, cow's, fluid, whole."
Click the checkbox on the "1 cup" line
Type the numeral 1 in the larger box.
Click on View Nutrients.
Scroll down until you see calcium. Notice that 1 cup of "Milk, cow's, fluid, whole" contains 276 mg of calcium
Scroll back to the top of the page and click on Current search.
Click on the food code number 11111150 with the description "Milk, calcium fortified, cow's, fluid, whole."
Click the checkbox on the "1 cup" line
Type the numeral 1 in the larger box.
Click on View Nutrients.
Scroll down until you see calcium. Notice that 1 cup of "Milk, calcium fortified, cow's, fluid, whole" contains 1,018 mg of calcium, or more than 3.5
times the calcium of the same amount of "Milk, cow's, fluid, whole."
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Step 3: Search by food code
Every description of a food in the What's in the Foods You Eat search tool is associated with the food's 8-digit USDA food code number. As discussed
in Task 2: "Describe the USDA Food Coding Scheme" of this module, the first digit of the food code identifies one of nine major food groups. The
second, third, and sometimes fourth digits specify increasingly more specific subgroups.
You may want to use the What's in the Foods You Eat search tool to explore the database by food code, in order to see the wide variety of foods that
are available in the search tool. For example, if you enter 924 in the search box, you will find 36 food codes that begin with those numbers (note that
this example is based on FNDDS 2.0 and future versions may yield slightly different results). The food items associated with these beginning digits
include soft drinks, carbonated beverages, colas, and similar beverages.
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Task 5: Key Concepts About the MyPyramid Equivalents Database
The MyPyramid Equivalents Database (MPED) is a tool to help researchers examine dietary recall data in terms of MyPyramid-defined food groups
and portion units. All foods in the FNDDS identified by food codes (e.g., lasagna) have been disaggregated into their component ingredients (e.g.,
pasta, mozzarella, beef) and these ingredients have been converted to equivalent amounts of relevant MyPyramid food groups (e.g., grains, milk,
meat). MPED contains data for all USDA survey food codes available for use with any nationwide dietary intake survey conducted from 1994 onward.
MyPyramid replaced the Food Guide Pyramid in January 2005. MPED, which was released in late 2006, supersedes all versions of its predecessor,
the Pyramid Servings Database.
MyPyramid uses cups and ounces as the standard portion units. It identifies equivalent amounts (i.e. “cup equivalents” and “ounce equivalents”) of
those measures for foods in the USDA databases. These equivalents are used in coding and processing food intakes from nationwide dietary
surveys in order to standardize the foods in the group to a nutritionally equivalent metric. Note that the Pyramid Servings Database data were in
terms of number of “servings.”
Data in MPED are of two general types: food data and intake data.
Food data
These data provide the number of MyPyramid Equivalents per 100 grams of food for the 32 MyPyramid food groups listed in the table below with their
respective measures. Food mixtures are separated into their ingredients before MyPyramid Equivalents are calculated.
MyPyramid Food Groups and Measures
MyPyramid Food Group Measure
Total grain Ounce equivalents
Whole grain
Non-whole grain Cup equivalents
Total vegetables Ounces cooked lean meat
Dark-green vegetables
Orange vegetables Ounce equivalents of lean meat
White potatoes Cup equivalents of vegetables or ounce equivalents of
Other starchy vegetables meat
Tomatoes Grams
Other vegetables Teaspoon equivalents
Total fruits Number of alcoholic drinks
Citrus fruits, melons, and berries
Other fruits
Total milk products (milk, yogurt & cheese)1
Milk
Yogurt
Cheese
Meat, poultry, fish
Meat (beef, pork, veal, lamb, game)
Organ meats (meat, poultry)
Frankfurters, sausage, luncheon meats (made from meat or
poultry)
Poultry (chicken, turkey, other)
Fish and shellfish high in omega-3 fatty acids
Fish and shellfish low in omega-3 fatty acids
Eggs
Soybean products (tofu, meat analogs)
Nuts and seeds
Cooked dry beans and peas2
Discretionary oil
Discretionary solid fat
Added sugars
Alcohol
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1 Formerly called Total Dairy.
2 Cooked dry beans and peas may be counted in either the vegetables group or the meat and beans group. Note that cooked dry beans and peas
should not be counted in both groups at the same time.
To illustrate, the food code 11542100, "Carry-out milk shake, chocolate," has values of .343 cup equivalents of total milk (milk, yogurt & cheese), .343
cup equivalents of milk, .631 grams of discretionary oil, 2.986 grams of discretionary solid fat, and 3.327 teaspoon equivalents of added sugars. This
food code has zero values for all other food groups because this food contains no MyPyramid equivalents for those groups.
Intake data
These data represent the number of MyPyramid equivalents provided by each food eaten per individual and daily totals per individual. They are
derived by applying MPED food data to intake data. Modification codes are also considered in the derivation of intake data in the MPED.
MPED includes data for the 1994-96 and 1998 Continuing Survey of Food Intakes by Individuals (CSFII) and is updated to include new information for
each 2-year release of NHANES data starting in 1999-2000.
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Task 5: How to Access the MyPyramid Equivalents Database
Step 1: Locate MyPyramid Equivalents Database
Go to the FSRG's website at http://www.ars.usda.gov/ba/fsrg.
Click on the My Pyramid Equivalents Database link.
Scroll down to the MyPyramid Equivalents Database for USDA Survey Food Codes section and click on the download link of the version that you are
interested in. This will take you to the MyPyramid Equivalents Product Downloads page that lists the downloadable files and specifies their contents.
Step 2: Download and review files for MyPyramid Equivalents Database
Download the complete set of files. Note that after the files are extracted, each download will include a documentation file in PDF format and a
"readme.txt" file in ASCII format. Read the "readme.txt" file (using any ASCII text viewer (such as Notepad or Microsoft Word) for information on what is
included in the database.
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Overview of NHANES Survey Design and Weights
Purpose
NHANES uses a complex sampling design and constructs sample weights to produce nationally representative data. Learning
about the features of the NHANES survey design and weighting will help ensure that the results of your analyses represent
unbiased estimates with accurate statistical significance levels.
Task 1: Explain NHANES Survey Design
NHANES data are obtained through a complex, multistage, probability sampling design that selects participants who are
representative of the civilian, non-institutionalized U.S. population. Participants are not selected by a simple random sample.
Over-sampling of certain population subgroups is done to increase the reliability and precision of health status indicator
estimates for those groups. Failing to account for the sampling design in your analyses may result in biased estimates and
overstated significance levels.
Key Concepts about the NHANES Survey Design (/nchs/tutorials/Dietary/SurveyOrientation/SurveyDesign/Info1.htm)
Task 2: Explain NHANES Sample Weights
NHANES has constructed various sample weights for single 2-year survey cycles to take into account survey non-response,
over-sampling, post-stratification, and sampling error. This task describes how sample weights are constructed in NHANES.
Due to the way NHANES participants are selected, sample weights always must be used to produce an unbiased national
estimate.
Key Concepts about the NHANES Sample Weights (/nchs/tutorials/Dietary/SurveyOrientation/SurveyDesign/Info2.htm)
IMPORTANT NOTE
For additional information on these topics, please see the Continuous NHANES Web Tutorial Weighting module (/nchs/tutorials
/Nhanes/SurveyDesign/Weighting/intro.htm) .
Page last updated: May 3, 2013
Page last reviewed: May 3, 2013
Content source: CDC/National Center for Health Statistics
Page maintained by: NCHS/NHANES
Centers for Disease Control and Prevention 1600 Clifton Road Atlanta, GA 30329-4027, USA
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Task 1: Key Concepts About the NHANES Survey Design
The NHANES sample is designed to be nationally representative of the civilian, non-institutionalized U.S. population, in
that it does not include persons residing in nursing homes, institutionalized persons, or U.S. nationals living abroad. Thus,
for NHANES 1999-2010, each year's sample and any combination of samples from consecutive years comprise a
nationally representative sample of the resident, non-institutionalized U.S. population.
In order to have sufficient sample sizes to obtain stable estimates for population subgroups of common interest, NHANES
data are released in 2-year cycles. Just as each year's sample is representative of the resident, non-institutionalized
U.S. population, the same is true for these 2-year cycles of data.
Sampling Design
NHANES data are not obtained using a simple random sample. Rather, a complex, multistage, probability sampling
design is used to select participants. The NHANES sampling procedure consists of 4 stages, shown and described below.
Four Stages of NHANES Sampling Procedures
Stage 1: Primary sampling units (PSUs) are selected from strata defined by geography and proportions of minority
populations. These are mostly single counties or, in a few cases, groups of contiguous counties selected with
probability proportional to a measure of size (PPS). Most strata contain two PSUs. Additional stages of sampling are
performed to select various types of secondary sampling units (SSUs), namely the segments, households, and
individuals that are selected in Stages 2, 3, and 4.
Stage 2: The PSUs are divided into segments (generally city blocks or their equivalent). As with each PSU, sample
segments are selected with PPS.
Stage 3: Households within each segment are listed, and a sample is randomly drawn. In geographic areas where
the proportion of age, ethnic, or income groups selected for over-sampling is high, the probability of selection for
those groups is greater than in other areas.
Stage 4: Individuals are chosen to participate in NHANES from a list of all persons residing in selected households.
Individuals are drawn at random within designated age-sex-race/ethnicity screening sub-domains. On average, 1.6
persons are selected per household.
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IMPORTANT NOTE
Further details about the sampling plan for NHANES 2003-2004 can be found in the 2003-2004 Interviewer Procedures
Manual.
It is important to note that pregnant women are sampled slightly differently during this stage. From 1999-2006, all women
of childbearing age (15-39 years of age) were asked during the screening interview whether they were pregnant and a
supplementary sample of pregnant women were included in the sample.
NHANES' unique sampling design has consequences for two issues that are critical to conducting accurate dietary data
analyses – the use of sample weights and special procedures for variance estimation.
The Use of Sample Weights
To make a collection of participants selected under the complex NHANES survey design represent the US non-
institutionalized civilian population, each sampled person is assigned a numerical sample weight. This measures the
number of people in the population represented by that particular sampled person. Sample weights for NHANES
participants incorporate adjustments for unequal selection probabilities and certain types of non-response, as well as an
adjustment to independent estimates (called control totals) of population sizes for specific age, sex, and race/ethnicity
categories. Sample weights must be used to obtain correct national estimates from the NHANES data. For more
information about the construction and use of sample weights in NHANES, see Task 2 in this tutorial or the Survey Design
Factors course in the Continuous NHANES Web Tutorial.
Procedures for Variance Estimation
The multistage, probability sampling design of NHANES means that individuals are selected as part of groups defined by
the strata and by the primary and secondary sampling units, rather than as specific individuals. Although this makes
NHANES data collection more efficient, it means that, in statistical terms, the sampling variance of NHANES estimates is
not based on the counts of individuals, but instead on the counts of the groups. Special variance estimation techniques
(implemented in several major statistical analysis software packages, such as SUDAAN, SAS Survey, and STATA) that
account for the sampling design must be used to obtain correct measures of sampling variance.
IMPORTANT NOTE
For more information about variance estimation, please see the Estimate Variance and Analyze Subgroups module of the
Basic Dietary Analyses course (coming soon).
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Task 2: Key Concepts About the NHANES Sample Weights
Each sampled person in NHANES is assigned a numerical sample weight that measures the number of people in the
population represented by that specific person. Sample weights for NHANES participants incorporate adjustments for
unequal selection probabilities and certain types of non-response, as well as an adjustment to independent estimates
(called control totals) of population sizes for specific age, sex, and race/ethnicity categories. These adjustments are made
at the aggregate level for an NHANES sample, so that estimates computed from that sample are nationally representative.
Because not all sampled persons completed all portions of the survey, each individual represented in a public release data
file may have several different sample weights assigned, depending on the nature of the non-response adjustments
required.
Two sets of sample weight variables are included in the demographics data file, an interview weight and an exam weight.
Other sets of sample weight variables are included in the files for the 24-hour dietary recall data – a day one dietary weight
and (for 2003-04 and later surveys) a two-day dietary weight. For most dietary analyses, a user should use one of these
versions (see “Choosing the correct weight” section). However, there are special subsamples that have their own sets of
sample weights that are different from the main four.
The set of all individuals that have nonzero values for a particular version of sample weights comprise a nationally
representative sample, so long as those sample weights are incorporated into statistical analyses. Performing append and
merge operations to pull together data elements from many data files may result in missing or zero values for sample
weight variables, so it is important to ensure that the data set for a particular analysis does not include individuals with zero
or missing values for the desired version of sample weight.
IMPORTANT NOTE
Due to the way NHANES participants are selected, sample weights must always be used to produce an unbiased national
estimate.
Sample weights are assigned in three steps (for more detailed information about these steps, see the Sample Design
module in the Continuous NHANES Web Tutorial):
1. the base weight is calculated;
2. adjustments for unit non-response are made; and
3. adjustment to population sizes (control totals) for selected age, gender, and race/ethnicity groups are made.
1. Calculating the base weight
In general a sample person is assigned a weight that is equivalent to the reciprocal of his/her probability of selection.
In other words:
However, calculating the base weight for a sample person in NHANES is much more complicated due to the survey's
complex, multistage design. In NHANES, the following equation, which takes into account the survey design, is used
to determine the base weight for a sample person:
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where
2. Adjusting for unit non-response
In NHANES, an individual may be broadly categorized as a non-respondent to two major components of the survey - .the
in-home interview and the MEC exam. An individual is considered a non-respondent to the interview if he/she was selected
to be in the sample, but did not participate in the in-home interview. Similarly, an individual who agreed to complete the
interview but did not agree to, or did not come in for, the MEC portion of the survey is considered a non-respondent to the
exam. Adjustments made for these types of survey non-response account only for sample person interview or exam non-
response, but not for component/item non-response (i.e., a sample person declined to have their blood pressure measured
in the examination component but completed all other examination components).
The base weights were adjusted for non-response to the in-home interview when creating interview weights and further
adjusted for non-response to the MEC exam when creating exam weights.
Most individuals in the NHANES have nonzero interview and nonzero exam weights. These two sample weight versions
are the most commonly used for statistical analyses, but there are two more versions of sample weights, used specifically
for dietary analyses, that incorporate additional non-response adjustments.
At the MEC interview, each individual was asked to complete a 24-hour dietary recall. An individual who did not agree to
complete the recall, or who provided unreliable data for the recall, is considered a non-respondent for the Day 1 dietary
recall. In addition to adjustments for interview and MEC exam non-response, the base weights were adjusted for non-
response to the Day 1 dietary recall when creating the Day 1 dietary recall weights.
For 2003-04 and later surveys, each individual who completed the 24-hour dietary recall in the MEC was scheduled to
complete a second recall at a later date by telephone. An individual who did not agree to complete the second recall, or
who provided unreliable data for the recall, is considered a non-respondent for the second dietary recall. In addition to
adjustments for interview, MEC exam, and Day 1 dietary recall non-response, the base weights were adjusted for non-
response to the second dietary recall when creating the two-day dietary recall weights.
Special subsamples
Statistically defined (or random) subsamples of the participants who participated in the MEC exams are asked to
participate in a variety of survey components. These include a variety of lab, nutrition/dietary, environmental, or mental
health components. (Please see the respective survey protocol/documentation for more specific information.) For example,
approximately one-half of participants are selected to give a fasting blood sample on the morning of their MEC exams. The
subsamples selected for these components are chosen at random with a specified sampling fraction (for example, 1/2 or
1/3 of the total examined group) according to the protocol for that component. Each component subsample has its own
designated sample weight, which accounts for the additional probability of selection into the subsample, as well as the
additional non-response.
3. Adjustment to population control totals
In addition to accounting for sample person non-response, weights are also post-stratified to match the population control
totals for each sampling subdomain. This additional adjustment makes the weighted counts the same as an independent
count of the Current Population Survey (CPS) of the U.S. Census.
IMPORTANT NOTE 2/3
Please see CPS website for more information: http://www.bls.gov/cps/home.htm.
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For the Day 1 and two-day dietary weights, an additional post-stratification step was performed to balance recalls across
days of the week.
Weights for over-sampled subgroups
The average sample weight within specific subgroups of the overall population may vary because of over-
sampling. For certain subgroups of particular public health interest, the proportion of individuals in the NHANES sample
is larger than the corresponding proportion in the U.S. population. This over-sampling iincreases the reliability and
precision of estimates of health status indicators for these population subgroups. Weighting schemes allow estimates from
these subgroups to be combined to obtain a national estimate that reflects the relative proportions of these groups in the
population as a whole.
Examples of over-sampled subgroups in surveys since 1999 include:
African Americans
Mexican Americans
Low-income White Americans (beginning in 2000)
Adolescents aged 12-19 years
Persons age 60+ years
Different subgroups have been over-sampled in other survey years. For example, during the late 1960s and early
1970s, there was concern that people of very low income and women of childbearing age were at greater risk of
malnutrition than the general population. Therefore, during the first National Health and Nutrition Examination Survey
(NHANES I), conducted in 1971-74, these subgroups were over-sampled.
IMPORTANT NOTE
For your own analyses, it is critical to carefully review the General Data Release Documentation, found on the home page
of each survey cycle, to determine which subgroups are over-sampled.
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