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Primary sampling unit (PSU) for variance estimation


PSU is a 3-digit numeric variable.

000: Not in Universe


PSU is the primary sampling unit variable that represents the impact of the sample design clustering on the estimates of variance and standard errors. It is constant within a sample design period and changes between sample design periods. For analysis, researchers need to use PSU in conjunction with STRATA to account for stratification and clustering when computing variance estimates with IPUMS NHIS data.

See the User Notes on variance estimation for additional information.

After the 2019 redesign, there is no longer a sampling weight to produce household population estimates. See the user note on the calculation and use of sampling weights for additional information.


This variable is completely comparable prior to 2019.

The NHIS questionnaire was substantially redesigned in 2019 to introduce a different data collection structure and new content. For more information on changes in terminology, universes, and data collection methods beginning in 2019, please see the user note.


  • 1963-2018: All households.
  • 2019-2022: Sample adults age 18+ and sample children age 0-17.


  • 1963-2022