User Notes

NHIS Sample Design

The National Health Interview Survey (NHIS) is a household, face-to-face health survey of approximately 100,000 people in 40,000 households, on average, each year. The NHIS sample is designed to be representative of the civilian, non-institutionalized population living in the United States. By definition, the NHIS excludes residents in long-term care facilities, active-duty Armed Forces personnel, and U.S. nationals living abroad. The NHIS has been fielded annually since 1957, making it the longest-running national health survey in the United States. Data collection is carried out continuously throughout the year, producing nationally representative samples each quarter and attenuating any potential seasonal biases.

The NHIS is a complex, multistage probability sample that incorporates stratification, clustering and oversampling of some subpopulations (e.g., Black, Hispanic, and Asian in later years). Every 10 years, the NHIS undergoes a sample redesign. Each new design period represents either an update to the previous design or a complete redesign. While each design period incorporates new information based on the most recent decennial census data, new geographic designations (MSA definitions), or new sampling procedures to improve efficiency, the general design of the NHIS has remained the same over time.

The first stage of sampling involves dividing the U.S. into approximately 1,900 geographically defined areas called Primary Sampling Units (PSU), most of which comprise a metropolitan area, a large county, or a group of smaller adjacent counties. PSUs are grouped into strata using social and demographic characteristics of the area. Depending on the year, one or more PSUs are sampled per stratum, with the probability of selection for each PSU being proportional to its population size (PPS) within strata. Large metropolitan areas are selected with certainty into the sample and are called self-representing PSUs.

In the second stage of sampling, a selection of geographic area segments is sampled from within each PSU. These segments are then subdivided into clusters, each of which contains a small number (approximately 4-9) housing units. All households are then assigned a quarter (of the year) for interview and are subsequently distributed across the 13 weeks of each quarter. All eligible members of these sampled housing units are invited to participate in the basic survey interview. Some household members additionally participate in supplemental interviews, either through random selection or based on answers to questions in the basic/core interview.

Beginning with the 1985 sample redesign, the NHIS included an oversample of the Black population to increase the reliability of estimates for this group. In 1995, the NHIS also implemented an oversample of Hispanics. Beginning with 2006, the NHIS redesign implemented an oversample of the Asian population. Due to this oversampling, many more Blacks, Hispanics, and now Asians are interviewed than would be if the sample were exactly proportional to the U.S. population. Thus, each person from these oversampled groups represents a smaller number of individuals than do other persons in the sample. The use of SAMPLING WEIGHTS, discussed in another user note, corrects for this oversampling to yield representative population estimates.

Along with sample redesign, the NHIS underwent a major survey redesign in 1997. One feature of this survey redesign merits emphasis: the random selection of a single adult and a single child to answer a battery of additional questions. While sampling one adult was a common practice in earlier supplements, earlier waves of the core questionnaire collected full information on all household members. After 1997, interviewers continued to collect information on the household, socio-demographic characteristics, and basic indicators of health status, disability, and utilization of health care services for all persons. However, to reduce interview length and biases from proxy reporting, a new sampling scheme for the Basic Module questionnaire was adopted, in which more extensive information was collected on one randomly selected sample adult and one sample child from each family. These questions appear in the sample adult and sample child files in the original NHIS public use files.   In IHIS, for variables based on questions asked of sample adults and/or sample children, the universe statement in the variable description refers to "sample adults" and/or "sample children" rather than "persons."

Additional Information

For additional information on each of the NHIS redesigns, users can access original NCHS documentation through links provided below.

1969-1974
National Center for Health Statistics. (1975). Health Interview Survey Procedure 1957-1974. Vital Health Stat, 1(11).
http://www.cdc.gov/nchs/data/series/sr_01/sr01_011acc.pdf

1975-1984
National Center for Health Statistics. (1985). The National Health Interview Survey Design, 1973–84, and Procedures, 1975-83. Vital Health Stat, 1(18).
http://www.cdc.gov/nchs/data/series/sr_01/sr01_018acc.pdf

1985-1994
National Center for Health Statistics. (1989). Design and estimation for the National Health Interview Survey, 1985-94. Vital Health Stat, 2(110).
http://www.cdc.gov/nchs/data/series/sr_02/sr02_110.pdf

1995-2005
National Center for Health Statistics. (1999). National Health Interview Survey: Research for the 1995–2004 redesign. Vital Health Stat, 2(126).
http://www.cdc.gov/nchs/data/series/sr_02/sr02_126.pdf

National Center for Health Statistics. (2000). Design and estimation for the National Health Interview Survey, 1995–2004. Vital Health Stat, 2(130).
http://www.cdc.gov/nchs/data/series/sr_02/sr02_130.pdf