What does “universe” mean in the IPUMS-International 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. In some instances the universe suggested by the census questionnaire is not accurate, however, because of post-enumeration data processing. IPUMS-International empirically verifies universes to obtain the most accurate statement possible of the universe. In some cases, there is no independent information in a sample to verify a universe.
Cases that are outside of the universe for a variable are labeled “NIU (not in universe)” on the codes page. Differences in a variable’s universe across samples are a common data comparability issue.
The universes will not always be free 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” category. But until we perform comprehensive data editing and allocation, we do not 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.
In our early data releases we performed some editing to clean up the universes, but that approach was not comprehensive enough to ensure we were properly identifying the variable with faulty information. We removed those universe edits for the December 2006 data release.