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.
- 1963-2018: All households.