- NCHS Division of Health Interview Statitistics
The National Health Interview Survey (NHIS) is the principal source of information on the health of the civilian noninstitutionalized population of the United States. The NHIS is one of the major data collection programs of the National Center for Health Statistics (NCHS). The National Health Survey Act of 1956 provided for a continuing survey and special studies to secure accurate and current statistical information on the amount, distribution, and effects of illness and disability in the U.S. and the services rendered for or because of such conditions. The survey referred to in the Act, now called the National Health Interview Survey, was initiated in July 1957. Since 1960, the survey has been conducted by NCHS, which was formed when the National Health Survey and the National Vital Statistics Division were combined.
II. PURPOSE AND SCOPE
The objective of the survey is to address major current health issues through the collection and analysis of data on the civilian noninstitutionalized population of the U.S. National data on the incidence of acute illness and injuries, the prevalence of chronic conditions and impairments, the extent of disability, the utilization of health care services, and other health-related topics are provided by the survey. A major strength of this survey lies in the ability to display these health characteristics by many demographic and socioeconomic characteristics.
The NHIS data are obtained through personal interviews with household members. Interviews are conducted each week throughout the year in a probability sample of households. The interviewing is performed by a permanent staff of interviewers employed by the U.S. Bureau of the Census. Data collected over the period of a year form the basis for the development of annual estimates of the health characteristics of the population and for the analysis of trends in those characteristics.
The survey covers the civilian noninstitutionalized population of the U.S. living at the time of the interview. Because of technical and logistical problems, several segments of the population are not included in the sample or int the estimates from the survey. Persons excluded are: patients in long-term care facilities; persons on active duty with the Armed Forces (though their dependents are included); and U.S. nationals living in foreign countries.
III. SAMPLE DESIGN
The NHIS is a cross-sectional household interview survey. Sampling and interviewing are continuous through each year. The sampling plan follows a multistage area probability design that permits the representative sampling of households. The first stage consists of a sample of about 200 primary sampling units (PSUs) drawn from approximately 1,900 geographically defined PSUs that cover the 50 States and the District of Columbia. A PSU consists of a county, a small group of contiguous counties, or a Metropolitan Statistical Area. Within PSUs, intermediate stage units called segments are defined in such a manner that each segment contains approximately 40 households. Within these segments, a systematic sample of eight households is selected for the NHIS sample.
The NHIS sample implemented with the 1985 data collection year was a complete redesign from previous years. A feature that was added for the 1985 design is the formation of panels of PSUs. The total NHIS sample of PSUs is subdivided into four separate panels such that each panel is a representative sample of the U.S. population. This design feature has a number of advantages, including flexibility for the total sample size. The 1985 NHIS sample included three out of the four panels and the 1986 NHIS sample includes two panels.
Another design feature implemented in 1985 is the over-sampling of black persons to improve the precision of estimates for that population. This resulted in an increase in the number of black persons in the NHIS sample by approximately 75 percent and an increase in the precision of most related statistics by more than 20 percent. The new sample design also facilitates followup studies of respondents and linkage with other national health-related data sets such as the National Death Index.
The households selected for interview each week are a probability sample representative of the target population. With four sample panels, data are collected from approximately 50,000 households including about 135,000 persons in a calendar year. Participation is voluntary and confidentiality of responses is guaranteed. The annual response rate of NHIS is over 95 percent of the eligible households in the sample. The response is divided equally between refusals and households where no eligible respondent could be found at home after repeated calls.
IV. DATA COLLECTION PROCEDURES
Data are collected through a personal household interview conducted by interviewers employed and trained by the U.S. Bureau of the Census according to procedures specified by NCHS.
All adult members of the household 17 years of age and over who are at home at the time of the interview are invited to participate and to respond for themselves. For children and for adults not at home during the interview, information is provided by a responsible adult family member (19 years of age and over) residing in the household. Between 65 and 70 percent of the adults 17 years of age and over are self-respondents. Generally, a random subsample of adult household members is selected to respond for themselves to questions on current health topics that are added each year.
Nationally, there are approximately 150 interviewers, trained and directed by health survey supervisors in each of the 12 Census Bureau Regional Offices. The supervisors are career Civil Service employees whose primary responsibility is the NHIS. The interviewers are part-time employees, selected through an examination and testing process. Interviewers receive thorough training in basic interviewing procedures and in the concepts and procedures unique to the NHIS.
Depending on the family size and the nature and extent of health conditions of family members, the length of interview ranges 20-90 minutes. On the average, the interviews require about 50 minutes in the household.
V. CONTENT OF THE QUESTIONNAIRE
The questionnaire consists of two basic parts: (1) a set of basic health and demographic items, and (2) one or more sets of questions on current health topics. The basic items constitute approximately 50 percent of the questionnaire and are repeated each year. These items provide continuous information on basic health variables. Questions on current health topics facilitate a response to changing needs for data and coverage of a wide variety of issues. This combination yields a unique national health data base.
The questionnaire includes the following types of basic health and demographic questions:
Demographic characteristics of household members, including age, sex, race, education, and family income.
Disability days, including restricted-activity and bed-disability days, and work- and school-loss days occurring during the two-week period prior to the week of interview, as well as 12-month bed days.
Physician visits occurring during the same two-week period, interval since the last physician visit, and the number of visits in the last 12 months.
Acute and chronic conditions responsible for these days and visits.
Long-term limitation of activity resulting from chronic disease or impairment and the chronic conditions associated with the disability.
Short-stay hospitalization data, including the number of hospital episodes during the past year and the number of days for each stay.
In addition, each of six representative subsamples is asked to respond to questions about one of six lists of selected chronic conditions.
Questions on special health topics change in response to current interest and need for data. The 1983 questionnaire contained questions on alcohol, dental care, physician services, and health insurance. The 1984 current health topic questionnaire was devoted entirely to issues of aging, 1985 covered health promotion and disease prevention, and 1986 includes questions on health insurance, dental health, vitamin and mineral intake, longest job worked, and functional limitations. The 1987 NHIS includes an extensive questionnaire on cancer risk factors and questions on child adoption.
Suggestions and requests for special health topics are solicited and received from many sources. These include university-based researchers, administrators of national organizations and programs in the private and public health sectors, and other parts of the U.S. Department of Health and Human Services such as the National Institutes of Health and the Centers for Disease Control. Topics are selected after consultation with agencies within the Public Health Service and after an assessment of priority health issues and the related need for population-based data. A lead time of at least 18 months is required to develop and pretest questions for new topics.