data collection for the dietary component; USDA is responsible for the design of the dietary data collection procedure, maintenance of the databases that are used to code and process the dietary intake information, and review and processing of the dietary information (see “Integrated NHANES and CSFII” below).
The NHANES has evolved from a periodic survey that originated in 1971, when the National Health Examination Survey was combined with the National Nutrition Surveillance System, to a continuously fielded survey. NHANES I data were collected for 1971-1975, NHANES II for 1976-1980, and NHANES III for 1988-1994. There was also an Hispanic Health and Nutrition Examination Survey (HHANES), which was conducted from 1982 through 1984. In 1999, NHANES became an ongoing survey, with detailed health, nutrition, and medical information collected from about 5,000 participants annually. Beginning with data for 1999-2000, NHANES findings have been released at 2-year intervals. To enable users to produce estimates from the NHANES public-use microdata files with sufficient reliability, sample weights are provided on a pooled basis for each 2 years’ worth of information. To protect confidentiality of respondents, not all variables are included in the public-use microdata files.
The sampling process is stratified and multistage: counties or groups of contiguous small counties are designated as primary sampling units (PSUs), and each year 15 PSUs are selected into the sample for household visits. Within PSUs, blocks or groups of blocks are selected, then households, and, finally, one or more individuals within households. The sample does not include people living in institutions or members of the Armed Forces. At sampled households, interviewers obtain the demographic characteristics of all household members, and one or more household members are selected for interview and examination by using fixed sampling fractions that distribute the sample into specific age-sex-race-ethnicity-income categories. If a child under the age of 6 is selected into the sample, then a proxy interview is conducted with the child’s primary caretaker. Interviews with children aged 6-11 years old are conducted through an assisted interview with a caretaker present. Blacks, Mexican-Americans, 12- to 19-year-olds, people aged 60 and over, pregnant women, and (beginning in 2000) people in low-income households are oversampled (see www.cdc.gov/nchs/data/nhanes/guidelines1.pdf [June 2005]).
In addition to demographic and economic background information and self-reported health status for each individual in the sample, physiological information, including precise measurements of height and weight