United States Decennial Census

United States Decennial Census
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Event information
Purpose	Population counting and apportionment of seats in the U.S. House of Representatives
Frequency	Every 10 years
Legal basis	U.S. Constitution (Article I, Section 2) and federal census statutes
Conducting agency	United States Census Bureau

The United States decennial census is a nationwide census conducted every 10 years to count the U.S. population and to apportion representation in the United States House of Representatives. The process is authorized by the U.S. Constitution and administered by the United States Census Bureau. In addition to apportionment, census results are used to support federal and state planning, including congressional districting and the distribution of public resources.

Constitutional and legal foundations

The decennial census is rooted in Article One of the United States Constitution, which directs a population count to determine the number of House seats allocated to each state. Federal law further establishes the procedures and timelines for conducting each census, including requirements for data collection and the delivery of results for apportionment purposes.

The census is conducted under a framework that emphasizes both accuracy and broad coverage. For example, the Census Bureau applies statistical methods and operational planning to count residents in ways intended to reflect where people actually live at the time of the census. The results support official representation decisions as well as subsequent administrative uses, including state-level electoral districting aligned with U.S. congressional district.

Apportionment and representation

A central outcome of the decennial census is the apportionment of seats in the House. After each census, the Census Bureau reports state population figures used to calculate the number of representatives each state receives. This process reflects the constitutional relationship between the national count and state representation, historically associated with House of Representatives.

Apportionment can change a state’s congressional representation and affects how political boundaries are drawn. States then use census-based counts in the redistricting process required for congressional elections under applicable state and federal law, which has been shaped by interpretations from Supreme Court of the United States and federal statutes governing voting rights and electoral administration, including Voting Rights Act of 1965.

Data collection and operational approach

Modern decennial censuses employ a combination of address listing, questionnaires, and follow-up operations. The Census Bureau uses nationwide address canvassing to build a comprehensive sampling frame and then invites households to respond. When responses are not received, enumerators conduct follow-up visits to attempt to obtain the required information.

The decennial census is also designed to support privacy and data stewardship requirements that govern how collected information may be used. The Census Bureau administers these protections under federal confidentiality rules associated with census participation, and it uses statistical sampling and related methods when complete enumeration is not feasible within operational constraints. As a result, the decennial census functions both as a headcount and as an integrated statistical production process.

Major historical milestones

The first United States Census was conducted in 1790 and set precedents for periodic population counting linked to representation. Over time, successive censuses expanded in scope and operational complexity as the country’s population grew and technologies improved. Many of the early censuses were conducted manually with forms and local procedures, while later censuses incorporated more systematic data processing approaches.

Notable policy and operational changes have occurred across multiple decades. In the mid-to-late 20th century, for instance, the Census Bureau increasingly relied on automated data processing and adjusted enumeration strategies in response to evolving administrative and technological capabilities. Contemporary censuses build on this history while addressing modern challenges such as population mobility and the need for accurate address-based frame construction.

Uses beyond apportionment

Beyond determining congressional seats, decennial census results are used for multiple purposes throughout the federal system and in state governance. Census data help guide redistricting, the allocation of resources, and planning for public services. Many programs use census-derived tabulations to measure population counts and demographic characteristics for administrative or funding eligibility.

The census also informs research and policy analysis by providing population statistics used by a wide range of institutions. Demographic outputs contribute to work in demography and support planning efforts across levels of government. Because the decennial census is conducted on a fixed 10-year schedule, it serves as a key anchor for tracking demographic change over time, including comparisons with American Community Survey data between censuses.