Introduction
The National Center for Health Statistics (NCHS), Division of Health Examination Statistics (DHES), part of the Centers for Disease Control and Prevention (CDC), has conducted a series of health and nutrition surveys since the early 1960's. The National Health and Nutrition Examination Surveys (NHANES) were conducted on a periodic basis from 1971 to 1994. Details of the design and content of each survey, and the public use data files are available (https://www.cdc.gov/nchs/nhanes.htm).
The most recent NHANES began in 1999. Every year, approximately 7,000 individuals, of all ages, are interviewed in their homes; of these, approximately 5,000 complete the health examination component of the survey. A majority of the health examinations are conducted in mobile examination centers (MECs); the MECs provide an ideal setting for the collection of high quality data in a standardized environment. In addition to the MEC examinations, a small number of survey participants receive an abbreviated health examination in their homes if they are unable to come to the MEC.
Target Population
The NHANES target population is the civilian, noninstitutionalized U.S. population. NHANES 1999-2000 includes over-sampling of low-income persons, adolescents 12-19 years, persons 60+ years of age, African Americans and Mexican Americans.
Survey Objectives
The major objectives of NHANES are:
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To estimate the number and percent of persons in the U.S. population and designated subgroups with selected diseases and risk factors,
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To monitor trends in the prevalence, awareness, treatment and control of selected diseases,
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To monitor trends in risk behaviors and environmental exposures,
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To analyze risk factors for selected diseases,
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To study the relationship between diet, nutrition and health,
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To explore emerging public health issues and new technologies;
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To establish a national probability sample of genetic material for future genetic research;
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To establish and maintain a national probability sample of baseline information on health and nutritional status.
NHANES Data Collection Procedures
The NHANES 1999-2000 data collection was carried out under a contractual agreement. First the eligible sample for the survey and performs tasks related to survey operations and data management are performed. The NHANES survey design is a stratified, multistage probability sample of the civilian noninstitutionalized U.S. population. The stages of sample selection are: 1) selection of Primary Sampling Units (PSUs) which are counties or small groups of contiguous counties; 2) segments within PSUs (a block or group of blocks containing a cluster of households); 3) households within segments; and 4) one or more participants within households. A total of 15 PSUs are visited during a twelve-month time period. A brief description of the data collection procedures follows.
NHANES Household Interview Data Collection Procedures
Initially, households are identified for inclusion in the NHANES sample and an advance letter is mailed to each address informing the occupant(s) that an NHANES interviewer will visit their home. The household interview component is comprised of Screener, Sample Person, and Family interviews, each of which has separate questionnaires (please refer to the data file codebooks). Trained household interviewers administer all of the questionnaires. In most cases, the interview setting was the survey participant's home. The interview data are recorded using the Blaise® computer-assisted personal interview (CAPI) system.
When the interviewer arrives at the home, he or she shows an official identification badge and briefly explains the purpose of the survey. If the occupant has not seen the advance letter, a copy is given to the occupant to read. The interviewer requests that the occupant answer a brief questionnaire to determine if any household occupants are eligible to participate in NHANES. If eligible individuals are identified, the interviewer proceeds with efforts to recruit these individuals. Initially, the interviewer explains the household questionnaires to all eligible participants 16 years of age and older, informs the potential respondents of their rights, and provides assurances about the confidentiality of the survey data (reiterating what is stated in the advance letter).
A majority of the household interviews are conducted during the first contact. If this is inconvenient for the survey participant, an appointment is made to administer the household interview questionnaires later. Household interviews for survey participants under 16 years of age are conducted with a proxy (usually the SP's parent or guardian). If there is no one living in the household who is over 16, participants under 16 years of age are permitted to self-report. Respondents are asked to sign an Interview Consent Form agreeing to participate in the household interview portion of the survey. For participants 16- 17 years of age a parent or guardian consents and the child gives his/her assent.
After the household interview is completed, the interviewer reviews a second informed consent brochure with the participant. This brochure contains detailed information about the NHANES health examination component. All interviewed persons are asked to complete the health examination component. Those who agree to participate are asked to sign additional consent forms for the health examination component. The interviewer telephones the NHANES field office from the participant's home to schedule an appointment for the examination. The interviewer informs the participants that they will receive remuneration as well as reimbursement for transportation and childcare expenses, if necessary.
Sources of the Questions Asked in NHANES 1999-2000
Many of the NHANES 1999-2000 questions were also asked in NHANES II, 1976-80, Hispanic HANES 1982-84, and NHANES III, 1988-94. New questions were added to the survey based on recommendations from survey collaborators, NCHS staff, and other interagency work groups.
Questionnaire Target Populations
There are different target population groups for the topics within and between NHANES questionnaire sections. For example, in the Nutrition and Diet Behavior section, questions pertaining to infant nutrition and breast-feeding were asked of proxy respondents for children 6 years of age and younger; alcohol consumption frequency questions were asked of persons 20+ years of age; and senior meal program participation questions were asked of respondents 60+ years of age. Data users should review the survey questionnaire codebooks thoroughly to determine the target populations for each NHANES questionnaire section and sub-section.
The NHANES Health Examination Component
When a participant arrives at the MEC, the MEC Coordinator greets the participant and verifies all pertinent identifier information. The participant is given a disposable paper gown, a pair of slippers, and a urine specimen cup ages 6 years and older. MEC staff direct survey participants to the rooms where the examination components are conducted. Each MEC survey team consists of one physician, one dentist, two dietary interviewers, three certified medical technologists, five health technicians, one phlebotomist, two interviewers and one computer data manager. Additionally there is a person designated as the Coordinators who is responsible for managing the movement of participants between examinations, remuneration and distributing a preliminary report of findings. Upon completion of the examination, each examinee is remunerated. Some of the medical findings of the examination are given to the examinees before they leave the MEC. The other reportable survey findings are mailed to participants after the laboratory assays and special tests are completed.
MEC Operations
Three MECs are equipped for use in NHANES. Each MEC consists of four large, inter-connected trailer units. An advance team sets up the MECs prior to the start of the survey examinations; water sewer, electrical, and communications lines are connected during set-up. The MEC equipment and data collection systems must be checked and calibrated prior to the start of survey data collection. The MECs are open a total of five days per week; the non-operational days change on a rotating basis so that appointments can be scheduled on any day of the week. Two examination sessions are conducted daily. For the convenience of the survey participants, appointments can be scheduled during morning, afternoon, or evening hours. The examinations require up to three hours to complete. At any given time during the survey, examinations are conducted at two survey locations simultaneously. Staff vacations are scheduled for periods of about one month at New Years and about two weeks during the summer; leaving ten and one-half months to conduct examinations.
Second Day Examinations and Dietary Interviews
Second day (i.e., repeat) MEC examinations were pilot tested in 1999 and implemented in 2000. Approximately five percent of examined persons over 20 years of age from each survey location are asked to complete second day exams. Second dietary interviews are completed on ten percent of all examined persons in 2000. The second day dietary interview is a primary data collection technique to improve the estimation of distributions of nutrient intakes. Second day examination and dietary data are not included in this data file.
Home Examinations
Participants under one year of age or 60+ years of age who are unable or unwilling to come to the MEC for an examination are eligible to receive a home examination. Home examination data are not included in this data file.
Guidelines for NHANES Data Users
NHANES 1999-2000 survey design and demographic variables are found in the DEMO.XPT file in this release. All of the data files can be linked by using the common survey participant identification number (variable name: SEQN). Merging information from multiple NHANES 1999-2000 data files using SEQN ensures that the appropriate information for each survey participant is linked correctly. All data files should be sorted by SEQN.
The NHANES 1999-2000 data files do not have the same number of records in each file. For example, there are different numbers of subjects in the Interview and Examination samples of the Survey. Additionally, the number of records in each data file varies depending on gender and age profiles for the specific component(s).
Confidential and administrative data were not included in this release. Some variables have been recoded or top-coded to protect the confidentiality of survey participants.
Sample Person Demographic File
The sample person demographic file is composed of a limited set of core variables that are required to analyze NHANES 1999-2000 data. Many variables that are listed in the Demographic questionnaire sections of the Household Interview were omitted from this data release due to concerns about participant confidentiality.
Demographic data file variables are grouped into three broad categories:
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Status Variables: Provide core information on the survey participant. Examples of the core variables include interview status, examination status, and sequence number. (Sequence number is a unique ID assigned to each sample person and is required to match the information on this demographic file to the rest of the NHANES 1999-2000 data.)
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Recoded Demographic Variables: The variables include age (age in months for persons through age 19 years, 11 months; age in years for 1-84 year olds, and a top-coded age group of 85+ years), gender, a race/ethnicity variable, an education variable, (< high school, high school, and more than high school education), country of birth (United States, Mexico, or other foreign born), and pregnancy status variable. Some of the groupings were made due to limited sample sizes for the two-year data set.
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Interview and Examination Sample Weight Variables: Sample weights are available for analyzing NHANES 1999-2000 data. Most data analyses require either the interviewed sample weight (variable name: WTINT2YR) or examined sample weight (variable name: WTMEC2YR). Use of the correct sample weight for NHANES analyses is extremely important. A good rule of thumb is to use "the least common denominator" approach. With this approach, the analyst checks the variables of interest. The variable that was collected on thesmallest number of persons is the "least common denominator," and the sample weight that applies to that variable is the appropriate one to use for that particular analysis.
- Subsample Data: There are no subsample data in this file.
Getting started with NHANES 1999-2000 data analysis
The National Health and Nutrition Examination Survey, 1999-2000 (NHANES 1999-2000) contains data for 9,965 individuals of all ages. Data were collected between March 1999 and December 2000. The data and corresponding documentation for the survey interview and examination components are found in several files.
NHANES data in this release are in SAS® transport file format. To access this data in any version of SAS, use the XPORT engine. It is recommended that you copy the transport files to a permanent SAS library. For example, assuming you have downloaded the Vision exam data to the folder "C:\NHANES", you can use the following SAS code to copy the Vision Exam Data:
LIBNAME XP XPORT "C:\NHANES\VIX.XPT";
PROC COPY IN=XP OUT=SASUSER;
RUN;