When census-takers can’t reach anyone at a particular address or obtain information about occupants in other ways, they sometimes use a last-resort statistical technique called “imputation” to fill in missing data. One marker of the quality of a census is how much it relies on imputation to add people to the count.
In the most extreme cases, census-takers have only an address taken from a master list drawn up in cooperation with local officials. They may not even know that a housing unit exists at that address, much less who lives there. If the address is indeed found to be an apparent dwelling place, the census-taker may not be able to get anyone to come to the door, and neither neighbors nor building managers may be willing and able to supply information. Yet the Census Bureau’s orders are to count everyone living in the U.S. on April 1, Census Day.
So to meet the goal of having a complete and accurate census, the Census Bureau imputes the existence and number of people living at the address in question, a procedure known as “count imputation.” (The other kind of imputation, called “characteristic imputation,” is when the Census Bureau has a head count for an address but is missing race, age or other personal information.) The number of imputed people tends to be higher among hard-to-count groups such as ethnic and racial minorities.
Read the full report Imputation: Adding People to the Census on the on the Pew Research Center's Social & Demographic Trends Web site.