Introduction
The National Center for Health Statistics (NCHS), Division of Health and Nutrition Examination Surveys (DHNES), 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. In 1999, NHANES became continuous. Every year, approximately 5,000 individuals of all ages are interviewed in their homes and complete the health examination component of the survey. The health examination is conducted in a mobile examination center (MEC); the MEC provides an ideal setting for the collection of high quality data in a standardized environment.
Details of the design and content of NHANES and the public use data files are available on the NHANES website.
Target Population
The NHANES target population is the noninstitutionalized civilian resident population of the United States. Over the years the NHANES design has changed periodically to sample larger numbers of certain subgroups of particular public health interest, thus to increase the reliability and precision of estimates of health status indicators for these population subgroups.
The primary sample design change for NHANES 2011-2012 is that there is an oversample of Non-Hispanic Asians in addition to the ongoing oversample of Hispanics, non-Hispanic Blacks, older adults, and low income whites/others. Since the total sample size in any year is fixed due to operational constraints, sample sizes for Hispanic persons and non-low income white and other persons were decreased in order to increase the sample sizes for Asians. Consequently, sample sizes for Mexican American Hispanic persons were also decreased compared to survey cycles prior to 2011.
The over-sampled subgroups in 2011-2012 survey were as follows:
- Hispanic persons;
- Non-Hispanic black persons;
- Non-Hispanic Asian persons
- Non-Hispanic white and Other* persons at or below 130 percent of the poverty level; and
- Non-Hispanic white and Other* persons aged 80 years and older.
* Other: Non-Hispanic persons reported races other than black, Asian, or white
Survey Objectives
The major objectives of NHANES are to:
- Estimate the number and percentage of persons in the U.S. population and in designated subgroups with selected diseases and risk factors;
- Monitor trends in the prevalence, awareness, treatment, and control of selected diseases;
- Monitor trends in risk behaviors and environmental exposures;
- Study the relationship between diet, nutrition, and health;
- Explore emerging public health issues and new technologies; and
- Provide baseline health characteristics that can be linked to mortality data from the National Death Index or other administrative records (e.g., enrollment and claims data from the Centers for Medicare & Medicaid Services).
Data Collection Procedures
NHANES uses a complex, multistage probability design to sample the civilian, noninstitutionalized population residing in the 50 states and D.C. Sample selection for NHANES followed these stages, in order:
- Selection of primary sampling units (PSUs), which are counties or small groups of contiguous counties.
- Selection of segments within PSUs that constitute a block or group of blocks containing a cluster of households.
- Selection of specific households within segments.
- Selection of individuals within a household.
In 2011-2012, 13,431 persons were selected for NHANES from 30 different study locations. Of those selected, 9,756 completed the interview and 9,338 were examined.
To facilitate the oversampling of the Asian population, survey materials were translated into Mandarin Chinese, both traditional and simplified, Korean, and Vietnamese. Recorded and written translations were also posted on the NHANES participants' web page, which included a short video that explained what was involved for the sample participant when participating in NHANES. This video was designed to promote interaction and to show sample participants some of the benefits to participating in NHANES. In addition to the above mentioned Asian languages, this video was also available in Amharic, French, Haitian Creole, Hindi, and Spanish.
Staff participated in cultural competency training to help them recognize and respect cultural differences. Local interpreters were hired when necessary and were provided with translated glossaries of terms, hand cards, and exam scripts to minimize errors in interpretation. In addition, a professional medical interpretation phone service was available to assist with any needs that were not otherwise provided for.
Contents of the 2011-2012 NHANES questionnaires are available on the NHANES website .
Procedures and protocols for each exam component in 2011-2012 can be found on the NHANES website.
Guidance for NHANES Data Users
We strongly encourage that all data users, prior to any analysis of the data, read all relevant documentations on the survey overall and for the specific data files to be used in their analysis. Specific data file documentation can be found via the link next to the respective data file on the NHANES website. Data users should also reference the 2011-2012 NHANES Analytic Guidelines [PDF - 108 KB] prior to beginning any analyses. This is available on the NHANES website.
Briefly, a few key points are discussed below.
NHANES 2011-2012 survey design and demographic variables are included in the Demographics file in this release. All of the NHANES public use data files can be linked by using the common survey participant identification number (variable name: SEQN). Merging information from multiple NHANES 2011-2012 data files using SEQN ensures that the appropriate information for each survey participant is linked correctly. All data files should be sorted by SEQN before merging.
The NHANES 2011-2012 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 are not released. Some variables have been recoded to protect the confidentiality of survey participants.
An additional race-ethnicity variable (RIDRETH3) is included in the NHANES 2011-2012 demographic dataset to reflect the change in the sample design for this survey cycle. RIDRETH1 is still included in the demographic dataset so that comparisons of NHANES 2011-2012 with previous survey cycles (without the Asian oversample) is possible. Refer to the Demographic Documentation and Analytic Guidelines for further detail.
Asians and Mexican Americans each comprise about 14 percent of the un-weighted examined sample for NHANES 2011-2012. This limited sample size means that detailed two and three way analytic comparisons of demographic subgroups will likely not meet all analytic criteria as outlined in the NHANES Analytic Guidelines. Therefore, more caution is needed when performing analyses of the NHANES 2011-2012 data for these population subgroups and four years (2011-2014) may be necessary for some analyses to be reliable. Please refer to the NHANES Analytic Guidelines, 2011-2012 [PDF - 108 KB] for more detailed information on analytic issues related to the use of 2011-2012 NHANES data.