Differences in the universe among years is the most common source of variable comparability problems. It is usually possible, however, to define a common universe by selecting the most restrictive criteria among the years in question. Numerous household variables are not available for persons living in units sampled as group quarters. Generally, these were large units such as institutions and boarding houses in which persons were sampled on an individual level. Some household-level items were not relevant to these persons.
Some items are only available for persons who answered one form or the other.
The variables in question are generally among the less important ones. These include notable universe differences, the wording of the question in given census years, and differing coding schemes among the public use microdata samples that present comparability issues. We include a M when differences are particularly great or where we thought specious comparisons were especially likely. A blank in a table for the census years from 1940 to the present ordinarily means that the variable was simply coded into broader categories. These are variables for which some of the original samples provided greater detail than others, but where this extra detail could be subsumed under a common set of more general categories.