The undercount of young children is one of the most vexing problems faced by the U.S. Census Bureau. The undercount rate for young children (ages 0 to 4) was not only high (5.4 percent) in the 2020 Census, but it has been increasing in every Census since 1980 when it was 1.4 percent.
The official young child net undercount rate in the U.S. Census increased from 4.6 percent in 2010 to 5.4 percent in 2020. But the change in the national rate masks a lot of changes among individual counties. Until now it has not been clear if the national increase in the young child net undercount rate between 2010 and 2020 was a product of a few large counties worsening, a large number of smaller counties worsening, or something else. This study addresses that question.