In early 2015, the Census Bureau assembled an Undercount of Young Children Research Team to pursue research on the undercount of young children (defined here as age 0 to 4) in the decennial census. Since then, researchers on this team have analyzed multiple existing datasets to learn more about the undercount of young children in preparation for the 2020 Census. This report summarizes key findings from these studies, which are listed in Appendix A. This report also discusses steps the Census Bureau has taken to improve the count of young children in the 2020 Census based on the research results.

The research adds to our knowledge of the characteristics of the young children who were most at risk of being missed or incompletely enumerated in the 2010 Census. No group of young children was immune from the risk of being missed in the census, but some groups were at higher risk than others. The results underscore the importance of examining young children separately from older children when studying coverage.

Young children with the highest risks of coverage errors include:

• Children who were not a biological or adopted child of the householder (i.e., grandchildren, other relatives, and children who were not related to the householder).
• Children who were Hispanic or racial minorities.
• Children living in complex households, defined as all households other than nuclear families, stem families (i.e., single-parent families), and single-person households.
• Children living in renter-occupied housing and multiunit structures.
• Very young children (those born in the few months prior to the census reference day).
• Children living in the largest and the smallest households.
• Children not enumerated by self-response.

There are multiple reasons why young children were missed in the 2010 Census. This research provided evidence of the following:

• Young children were missed because of different types of error, such as the housing unit was missed, the entire household was missed, or part of the household or just the child was missed. Whole-household errors were more common for biological and adopted children while partial-household errors were more common for grandchildren, other relatives, and nonrelatives of the householder.
• Young children were missed because they lived in hard-to-count households with enumeration challenges.
• Young children may have been missed along with their young mothers.
• Cooperative self-respondents made errors when they created household rosters. These often involved children who were not related to the householder or who were relatives other than biological and adopted children.