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Codes and Frequencies

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

Prior to 2019, Every child in each family, except for active duty armed forces members and emancipated minors, was eligible to be selected as the sample child. Emancipated minors are persons age 14 to 17 who are married or living with a partner OR who live on their own without the supervision of a parent, other adult family member, or legal guardian.

CSTATFLG has the value "no one selected as sample child" (code 4) for all children in the family in the following two cases prior to 2019: 1) if sample child status was incorrectly assigned to someone who was ineligible, or 2) if it was unknown which child in the family was chosen as sample child.

A similar flag variable, ASTATFLG, identifies the record of a sample adult.

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


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 under age 18.
  • 2019-2022: Sample children ages 0-17.


  • 1997-2022