The most recent NHANES began in 1999. Every year, approximately 7,000 individuals, of all ages, are interviewed in their homes and 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. The NHANES target population is the civilian, noninstitutionalized United States population. NHANES 1999-2000 includes over-sampling of low-income persons, adolescents 12-19 years of age, persons 60+ years of age, African Americans and Mexican Americans. 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 a separate questionnaire (please refer to the data file documentation). 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 them 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 their 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.
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. 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 Coordinator who is responsible for managing the movement of participants between examinations, providing 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 communication 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, and the nonoperational 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 5 percent of examined persons over 20 years of age from each survey location were asked to complete second day exams. Second day dietary interviews were 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 1 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.
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 the smallest 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.
NCHS June 2004 Version--NHANES Analytic Guidelines
Beginning in 1999, the National Health and Nutrition Examination Survey (NHANES) became a continuous, annual survey rather than the periodic survey that it had been in the past. For a variety of reasons, including disclosure and reliability issues, the survey data are released on public use data files every two years. Thus, the data release cycle for the ongoing (and continuous) NHANES is described as NHANES 1999-2000, NHANES 2001-2002, NHANES 2003-2004, etc. In addition to the analysis of data from any two-year cycle, it is possible to combine two or more "cycles" (e.g., 1999-2000 and 2001-2002) to create NHANES 1999-2002, thus increasing sample size and analytic options. In order to produce estimates with greater statistical reliability, combining two or more two-year cycles of the continuous NHANES is encouraged and strongly recommended. To facilitate analysis of these first two cycles of the continuous NHANES, the appropriate four-year sample weights have been calculated and added to the demographic data files for both 1999-2000 and 2001-2002. Thus, users of the earlier release of the NHANES 1999-2000 demographic file must use the updated demographic file to appropriately analyze the combined four-year data. These four-year sample weights have the same variable name in each two-year demographic file. For example, for the sample persons for whom there are MEC data items, the variable name for the four year weight is WTMEC4YR. The procedure for variance estimation (sampling errors) has been changed for NHANES 1999-2000 to one that protects confidentiality and allows the use of "PSUs" and is the recommended approach for analysis on the ongoing and continuous NHANES. This method creates Masked Variance Units (MVUs) which can be used as if they were Pseudo PSUs to estimate sampling errors (similar to past NHANES). The Pseudo-PSUs on the data file are not the "true" design PSUs. They are a collection of secondary sampling units aggregated into groups called Masked Variance Units (MVUs) for the purpose of variance estimation. They produce variance estimates that closely approximate the variances that would have been estimated using the "true" design variance estimates. These MVUs have been created for both NHANES 1999-2000 and NHANES 2001-2002 and added to the demographic data files for both two-year periods. They can also be used for the combined four-year dataset. The stratum variable is SDMVSTRA and the PSU variable is SDMVPSU. Software such as SUDAAN, STATA, and SAS can be used to estimate sampling errors by the Taylor series (linearization) method. Typically, the dataset should first be sorted by SDMVSTRA and SDMVPSU. There are no replicate weights provided for NHANES 2001-2002. Replication techniques can still be used to estimate sampling errors if the software, such as WESVAR, computes its own set of replicate weights based on the nested PSU within stratum design. Some data components of NHANES are only available for 1999-2000 or 2001-2002. For these components, the two-year sample weights and the two-year MVUs can be used for analysis. For 2001-2002, the two-year weights and MVUs are provided on the demographic data file. For 1999-2000, the previously released demographic file has been updated to add the MVUs and four-year sample weights. At this time, the preferred approach for calculating sampling errors is to use the MVUs and to ignore the JK-1 technique utilized as an interim approach with the release of the NHANES 1999-2000 data. On occasion, there may be a particular issue that requires comparison of results from NHANES 1999-2000 with NHANES 2001-2002. For summary statistics such as means and proportions, the appropriate two-year sample weights and MVUs can be used for reasonably valid inferences (although caution should be used when producing estimates for any detailed population subgroup). Calculation of totals (e.g., estimates of the number of persons with a particular characteristic such as BMI greater than 30 or with impaired glucose tolerance) is not appropriate for NHANES 1999-2000 unless the numbers are ratio-adjusted to population counts based on year 2000 Census figures. This is not an issue for NHANES 2001-2002 which was linked to the 2000 Census counts. Users are encouraged to check the NHANES Web site on a regular basis to be aware of the latest version of these NHANES analytic guidelines.
ICPSR data undergo a confidentiality review and are altered when necessary to limit the risk of disclosure. ICPSR also routinely creates ready-to-go data files along with setups in the major statistical software formats as well as standard codebooks to accompany the data. In addition to these procedures, ICPSR performed the following processing steps for this data collection:
NCHS provides continuous updates/new data notification, as well as other important information for the NHANES. It is recommended that users of these data sign up for the information through the NHANES Listserv. The "What's New" page on the NHANES Web site provides updates/new information which may not be included in the listserv emails. Further, not all documentaion files are included with this ICPSR release and may be found at the NHANES 1999-2000 Web site.
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