What does the census actually do?

The census is a constitutionally mandated count of every person residing in the United States, conducted every ten years since 1790. Its primary constitutional purpose is apportionment: dividing the 435 House seats among the states by population, which also determines each state's number of electoral votes. The resulting data then drives redistricting, since state and local district boundaries must be redrawn to reflect population changes. Beyond representation, census figures guide the distribution of hundreds of billions of dollars in annual federal funding for roads, schools, hospitals, and assistance programs, and they underpin an enormous amount of business, academic, and public planning.

Two facts matter for participation. Responses are confidential by federal law, which prohibits the Census Bureau from sharing individual data with any other agency, including immigration and law enforcement, for 72 years, and penalties for violation are criminal. And the count includes everyone residing in the country regardless of citizenship status, which reflects the constitutional language about counting persons and has been litigated repeatedly. Undercounting is the persistent problem, since it falls disproportionately on young children, renters, rural residents, and communities of color, and an undercount costs those communities representation and funding for a full decade. The Bureau also runs the American Community Survey continuously between decennial counts, which is where most of the detailed demographic and economic data that researchers and journalists cite actually comes from.