What does “universe” mean in the variable descriptions?
The universe is the population at risk of having a response for the variable in question. In most cases these are the households or persons to whom the census question was asked, as reflected on the census questionnaire. For example, children are not usually asked employment questions, and men and children are not asked fertility questions. Cases that are outside of the universe for a variable are labeled “NIU” on the codes page. Differences in a variable’s universe across samples are a common data comparability issue.
The universes will not always be entirely clean of apparently erroneous cases. Some persons or households that should not have answered the question did, and some that should have answered may be included in the “NIU” (not in universe) category. It is often not possible to know whether the variable in question is in error or whether the variables that define the universe (for example, age or employment status) are incorrect.