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Codes and Frequencies

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ASTATFLG identifies the record of a sample adult. A sample adult is the one person over age 18 per household who is randomly selected to complete the sample adult questionnaire. For 1997-2018, a sample adult is the one adult per family who was selected at random by the computerized survey instrument to answer additional health-related questions, under the survey design instituted in 1997. All persons other than sample adults (those not having the value 1 in ASTATFLG) are coded as "not in universe" for variables derived from the additional questions asked only of sample adults. (Some additional questions were asked of both sample adults and sample children.) Every adult in each family, except for active duty armed forces members, was eligible to be selected as the sample adult.

Prior to 2019, ASTATFLG has the value "no one selected as sample adult" (code 4) for all adults in the family in the following two cases: 1) sample adult status was incorrectly assigned to someone in the armed forces, or 2) it could not be ascertained which adult in the family was chosen as sample adult.

For 2020 only, SALNGPRTFLG further distinguishes between sample adults included in the longitudinal sample and sample adults included in the partial sample. Please see the user note on COVID-related changes to the NHIS for more information on the longitudinal and partial samples.

A similar flag variable, CSTATFLG, identifies the record of a sample child.

This variable is automatically included in all extracts, but only has valid values for 1997 forward.


Comparability is affected by changes in the definition of an "adult" over time and the 2019 questionnaire redesign. Prior to 2005, "adult" status was uniformly defined as being age 18 or older. From 2005 to 2018, the recognized age of adulthood for defining sample adults varied by state of residence. As the Field Representative's Manual for 2006 explains:

A sample adult is randomly selected by the computer from each family with a household member equal to or greater than the age of majority for a given state and asked more detailed health related questions. In most states this age is 18 years old, but in Alabama and Nebraska this age is 19 and in Mississippi it is 21.

For 2005-2018, persons age 18 in Alabama and Nebraska would be coded "not in universe" (IPUMS NHIS code 0) for ASTATFLG, as would persons age 18, 19, and 20 in Mississippi.

Questionnaire design changes introduced in 2019 limit comparability with earlier years. The NHIS questionnaire was substantially redesigned in 2019 to introduce a different data collection structure and new content. Notably, beginning in 2019, questions are asked only of sample adults and sample children. For more information on changes in terminology, universes, and data collection methods beginning in 2019, please see the user note.


  • 1997-2018: Persons age 18+.
  • 2019-2022: Sample adults age 18+.


  • 1997-2022