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National Health Interview Survey (NHIS)

National Center for Health Statistics

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Overview

Data from NHIS are used to monitor the health of the U.S. population on a broad range of health topics. During household interviews, NHIS collects information on the demographic and socioeconomic characteristics of respondents, in addition to information on activity limitation, illnesses, chronic conditions, health insurance coverage (or lack of), use of health care, and other health topics.

Coverage

The survey covers the U.S. civilian noninstitutionalized population. Among those excluded are patients in long-term care facilities, people on active duty with the armed forces (although their civilian family members are included), people who are incarcerated, and U.S. nationals living in foreign countries.

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Methodology

NHIS is a cross-sectional annual household interview survey conducted continuously throughout each year. NHIS uses a stratified, multistage sample design that permits the representative sampling of households and noninstitutional group quarters. The sample design is redesigned after every decennial census to better measure the changing U.S. population and to meet new survey objectives. A new sample design was implemented in 2016, with additional changes in 2018.

The current sampling process involves dividing the United States into 1,689 geographic areas. These geographic areas are defined as counties, county equivalents, or groups of counties, are usually contiguous, and do not cross state boundaries. Within some states, the geographic areas are divided into two strata defined by population density (generally, urban counties and rural counties). For the remaining states, all the geographic areas form one stratum. Within each stratum, clusters of about 2,500 addresses are defined; each is located entirely within one of the 1,689 originally defined geographic areas. Within each stratum, a specific number of clusters is systematically selected for the NHIS sample. The number selected is generally proportional to the number of clusters in the strata. The exception is in the 10 least populous states and the District of Columbia, where a slightly higher number of clusters are selected to ensure that all states have a minimum number of addresses in the sample.

In 2019, for the first time in more than 20 years, the NHIS questionnaire was redesigned to increase relevance, enhance data quality, and minimize respondent burden. Questionnaire content was revised to make overlapping content consistent with other federal health surveys, establish a long-term structure of ongoing and periodic topics, and incorporate advances in survey methodology and measurement. As part of the 2019 redesign, the number of people from whom data are collected per household was reduced to one “Sample Adult” age 18 and older and one “Sample Child” age 17 years and younger. The Sample Adult and Sample Child can be from separate families within the household. Information about the Sample Adult is collected from the Sample Adult unless they are physically or mentally unable to do so, in which case a knowledgeable proxy can answer. Information about the Sample Child is collected from a parent or adult who is knowledgeable about and responsible for the health care of the Sample Child.

In contrast, the 1997–2018 NHIS administered questions about the family separately for each family in the household. The family interview asked about the family as a whole and about each member of the family. Then, within each family, a Sample Adult and Sample Child were selected. From 1997 through 2018, NHIS also included a family questionnaire. Questions from the family questionnaire were answered by a member of the family on behalf of all members of the family.

Starting with 2019, the redesigned NHIS questionnaire consists of three modules: Household Roster, Sample Adult Interview, and Sample Child Interview. Before 2019, NHIS consisted of four modules: Household Composition, Family Core, Sample Adult Core, and Sample Child Core.

In 2020, data collection procedures were disrupted due to the COVID-19 pandemic. From April through June 2020, all interviews were conducted by telephone only. From July through December 2020, interviews were attempted by telephone first, with follow-ups to complete interviews in person if needed. In 2020, 70.7% of Sample Adult interviews (from the original 2020 sample) and 68.0% of Sample Child interviews were conducted at least partially by telephone, compared with 2019 when 34.3% of Sample Adult interviews and 31.7% of Sample Child interviews were completed by telephone. To address concerns about a possible loss of coverage and lower response rates associated with the change to telephone interviews, about one-half of the original adult sample allocated for August–December 2020 was replaced with Sample Adults who had completed the 2019 NHIS Sample Adult interview. About one-third of the 2020 NHIS Sample Adult interviews comprised Sample Adults previously interviewed for the 2019 NHIS.

From January through April 2021, interviews continued to be attempted by telephone first. Beginning in May 2021, interviewers returned to regular survey interviewing procedures: First contact with household members was attempted in person, with follow-up allowed by telephone. In 2021, 62.8% of the Sample Adult interviews and 61.4% of the Sample Child interviews were conducted at least partially by telephone.

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Sample Size and Response Rate

The annual NHIS sample size can be reduced or increased as needed. Since 2019, the NHIS sample design was expected to yield about 27,000 Sample Adult interviews and 9,000 Sample Child interviews in roughly 35,000 households each year. The annual sample size for both the 2016–2018 samples and the previous 2006–2015 sample design was about 35,000 households with about 87,500 people.

In the 2022 NHIS, 28,854 households were interviewed, resulting in data from 27,651 Sample Adults and 7,464 Sample Children. In 2022, the total household response rate was 49.6%. The 2022 final Sample Adult response rate was 47.7% and the final Sample Child response rate was 45.8%. In the 2021 NHIS, 30,673 households were interviewed, resulting in data from 29,482 Sample Adults and 8,261 Sample Children. In 2021, the total household response rate was 52.8%. The final Sample Adult response rate was 50.9% and the final Sample Child response rate was 49.9%. Due to impacts of the pandemic on data collection methodology, in the 2020 NHIS, 21,930 households were interviewed, resulting in data from 21,153 Sample Adults and 5,790 Sample Children. In 2020, the total household response rate was 50.7%. The final Sample Adult response rate was 48.9% and the final Sample Child response rate was 47.8%. In the 2019 NHIS, 33,138 households were interviewed, resulting in data from 31,997 Sample Adults and 9,193 Sample Children. In 2019, the total household response rate was 61.1%. The final Sample Adult and Sample Child response rates were both 59.1%.

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Issues Affecting Interpretation

Due to the 2019 redesign of the NHIS questionnaire and other changes in weighting and design methodology, estimates since 2019 may not be comparable with previous years.

A new approach to weighting was implemented with the 2019 redesign to address nonresponse bias. The updated approach for nonresponse adjustment used multilevel regression models that include paradata variables predictive of survey response and selected key health outcomes. In addition, the nonresponse adjusted weights were calibrated not only to U.S. Census Bureau population projections but also to an expanded set of sociodemographic and geographic variables from the American Community Survey (ACS). See Bramlett et al. for more information.

In 2020, data collection procedures were changed due to the COVID-19 pandemic, and response rates were lower. Modifications to the survey weighting methodology were implemented for 2020 to address some of the survey design differences, including the changes in the mode of data collection, the reduction in the original sample size, as well as others. See Dahlhamer et al. for more information.

Starting in 2021, nonresponse adjustment was made using Recursive Partitioning for Modeling Survey Data instead of using multilevel regression models. For more information, see: “National Health Interview Survey, 2021 Survey Description.”

Before 2019, the last NHIS questionnaire redesign was in 1997. As part of the 1997 questionnaire redesign, some basic concepts were changed, and other concepts were measured differently. For some questions, the reference period was changed. Also in 1997, the data collection methodology changed from paper-and-pencil questionnaires to computer-assisted personal interviewing. Some indicators presented in Health, United States begin with 1997 data because the redesign caused comparability issues. Moreover, starting with Health, United States, 2005, estimates for 2000–2002 were revised to use 2000-based weights, and they differ from previous editions of Health, United States that used 1990-based weights for those data years. The weights available in the public-use NHIS files for 2000–2002 are 1990-based. Data for 2003–2011 use weights from U.S. Census 2000. Data for 2012 and beyond use weights from U.S. Census 2010. In 2006–2010, the sample size was reduced, which is associated with slightly larger variance estimates than in other years when a larger sample was fielded. In 2010–2018, a geographic nonresponse adjustment was made to both the Sample Adult File and the Sample Child File weights; see Moriarity.

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References

 

For more information, see the NHIS website.

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