The variable HHTENURE in ATUS-X has 171 cases coded as NIU (not in universe). Howeover, this variable’s universe is CPS households. The ATUS sample is drawn from CPS households. So why are there NIU cases for the variable HHTENURE?
The universes in ATUS-X are not always entirely clean of apparently erroneous cases. Some people or households that should not have answered the question will have done so, and some that should have answered may be included in the “NIU” category. With the exception of the variables we construct for the convenience of users, these errors reflect problems in the original data. In the instance of HHTENURE, the issue is caused by missing values in the original ATUS data for these 171 NIU cases.
I hope this helps.