When U.S. Census Bureau workers couldn’t find out any information about some households after repeatedly mailing them questionnaire reminders and sending census takers to knock on their doors, the statisticians turned to an obscure, last-resort statistical technique known as “imputation.”
Less than 1% of households were counted using the technique during the 2020 census. But some conservative political groups are questioning it, potentially laying a foundation for legal challenges to the data that will ultimately be used for drawing congressional and legislative districts.
Imputation involves using information about neighbors with similar characteristics to fill in head counts or demographic characteristics for households lacking data. For instance, a rowhouse with no information may be counted as having two people if a neighboring rowhouse is occupied by two people. The technique tends to be utilized in hard-to-count places, often with racial and ethnic minorities, where people haven’t answered the census questionnaire and could otherwise go uncounted.
“It makes the overall dataset – or census in this case – more accurate than leaving the gaps blank,” Pat Cantwell, a bureau official, said in a blog post earlier this year. “By using imputation, we fill in what we don’t know, using information we do know.”
The focus for conservatives is on how this technique was applied to college dorms, nursing homes, prisons and other places where people live in groups. These residents were particularly difficult to count during the 2020 census because the pandemic sent college students fleeing campuses and put nursing homes in lockdown. In response, the Census Bureau unexpectedly decided to use the technique for group housing, where about 3% of the U.S. population lives.