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 survey question was asked, as reflected on the survey questionnaire. For example, employment variables do not include children, since the NHIS does not ask children about employment. The IHIS Project staff empirically verifies variable universes to obtain the most accurate statement possible. Cases that are outside of the universe for a variable are labeled “NIU” on the codes page. A change in a variable’s universe across years is a common data comparability issue. In some cases, IHIS imposes a different variable universe than the one found in the original NHIS public use data. Usually this is done to distinguish cases with meaningful zeros (e.g., adults who reported having individual incomes of zero dollars) from cases where “NIU” was also originally coded as zero (e.g., for children, who were not asked about their individual incomes)
