Demographer Steven Ruggles has been pointing the problems out for some time (see here and here). He’s quoted in a recent AP report that also notes:

Residents did not respond to a multitude of questions about sex, race, Hispanic background, family relationships and age, even when providing a count of the number of people living in the home, according to documents released by the agency. Statisticians had to fill in the gaps. …

The information is important because data with demographic details will be used for drawing congressional and legislative districts. That data, which the Census Bureau will release Thursday, also is used to distribute $1.5 trillion in federal spending each year.

The documents, made public in response to an open records request from a Republican redistricting advocacy group, don’t shed much light on why questions were left unanswered, though theories abound. [Including the theory that Trump is to blame!] …

If available records didn’t turn up the information needed, they turned to the statistical technique called imputation that the Census Bureau has used for 60 years. The technique has been challenged and upheld in courts after past censuses.

In some cases, statisticians looked for information answered about one member of a family, such as race, and applied it to another member that had blank answers. Or they assigned a sex based on the respondent’s first name. In other cases, when the entire household had no information, they filled it in using data of similar neighbors.

The courts may have upheld imputation in the past when 1-3% failed to answer the questions, but will they do so this time when 10-20% failed to do so? Stay tuned!