The Decennial Census of Population and Housing: An Overview

The Decennial Census of Population and Housing: An Overview

The Decennial Census of Population and Housing: An Overview

Updated April 23, 2025 (IF12909)

Every decade in the year ending with "0," the U.S. Census Bureau conducts the decennial census of population and housing. The decennial census is mandated by Article I, Section 2 of the U.S. Constitution, in order to determine each state's apportionment of seats in the House of Representatives. Under current law, the Census Bureau must inform Congress of the general subjects to be addressed three years prior to the decennial census, and of the actual questions to be asked two years prior to the decennial census (13 U.S.C. §141).

Data from the decennial census are used for many purposes, including to inform various policy decisions, guide federal funding, offer the public insights on population trends, and help the private sector make informed business decisions.

Due to the significance of the decennial census for apportionment in particular, Congress has historically shown interest in and provided oversight of the decennial census. This In Focus provides an overview of some of the issues involved in the decennial census, including funding, data collection, and testing. It concludes with a discussion of oversight opportunities for Congress.

2030 Decennial Census

The Census Bureau begins planning for an upcoming decennial census more than a decade prior to the census year. The Census Bureau has begun executing its planning timeline for the 2030 census provided in Figure 2.

In 2023, the Census Bureau established the 2030 Census Advisory Committee (CAC), comprised of members representing various stakeholder groups and interests, to review and provide feedback related to 2030 census plans. The Census Bureau has utilized similar advisory committees in the past. The 2030 CAC was terminated on February 28, 2025. Other aspects of plans for the 2030 decennial census, including field testing and technological innovations, are discussed throughout this In Focus.

Budget Trends

During each decade, annual funding levels for the Census Bureau have generally increased as the decennial April 1 "census day" draws near. Figure 1 shows this general increase in Census Bureau appropriations adjusted for inflation throughout the decades leading up to the 2000, 2010, and 2020 censuses; and Census Bureau appropriations leading up to the 2030 decennial census so far.

Figure 1. U.S. Census Bureau Funding FY1991-FY2024

Adjusted for inflation

Source: Office of Management and Budget, Analytical Perspectives: Budget of the U.S. Government Fiscal Years 1998-2025.

Notes: All values adjusted to constant dollars using the CPI Inflation Calculator from the base year in October to October 2024. Bar colors are arranged by decennial census cycle.

Data Collection

The primary methods for the Census Bureau to collect data from households are through self-response to a survey via the internet, through mail via a paper form, or by phone. To support data collection, the Census Bureau maintains a Master Address File (MAF), which is intended to contain an updated inventory of all known living quarters in the United States and Puerto Rico.

Figure 2. 2030 Census Planning Timeline

Source: U.S. Census Bureau, "2030 Census Planning Timeline."

Local Update of Census Addresses

Prior to a decennial census, tribal, state, and local governments may participate in the statutorily authorized Local Update of Census Addresses (LUCA) operation. LUCA allows these governments to review and comment on the residential addresses in their jurisdictions prior to the decennial census count. The Census Bureau has implemented some changes for the 2030 LUCA, including allowing early access to Census Bureau address counts, using email over postal service as the primary means of contact for participants, and allowing governments additional time to submit address list changes.

Administrative Data

The Census Bureau is authorized in statute to use administrative data. Examples of administrative data include (1) individual data, such as Social Security numbers and income tax return data; (2) business identifying information, such as descriptions of financial, ownership, and other characteristics; and (3) location data, such as latitude or longitude points, telephone numbers, and address information. Administrative data is used in the decennial census in various ways, including counting households that do not respond to the census. The Census Bureau found approximately 6% of addresses nationwide were enumerated using administrative data during the 2020 census.

Data Protection and Confidentiality

The Census Bureau uses a system known as disclosure avoidance, which is defined as "a process used to protect the confidentiality of respondents' personal information." For the 2020 decennial census, the bureau implemented a newer protection framework called differential privacy in which statistical noise was applied to respondents' data. While introducing noise to a data set protects the privacy of respondents, it also impacts data quality.

Collecting Responses

The 2020 decennial census was the first time internet-based responses were available. The Census Bureau found that nearly 80% of households responded online, 18% responded by mail, and 2% responded by phone. Group quarters, such as correctional facilities, university student housing, and military quarters, have responses submitted by the respective group quarters' administrators via online portal or mail. Overall, 67% of households self-responded.

For the remaining 33% of households that did not initiate a response, the Census Bureau conducted a nonresponse follow up (NRFU) operation in which enumerators interview households in person, collecting responses on mobile phones. Response to the decennial census is required by law.

Field Tests and Research

The Census Bureau has historically conducted field tests in the decade leading up to census day. Field tests are conducted to inform decennial census design by helping to refine operations, methodologies, and technologies used during enumeration. The 2019 field test was of particular interest to Congress. This field test addressed the effect of a proposed citizenship question on self-response rates.

The Census Bureau has begun conducting ongoing small-scale testing for the 2030 census that aims to examine how people respond to the online questionnaire and the invitation materials they receive to complete the survey, with more testing expected.

Two larger field tests are planned for later in the decade. This includes the 2026 census test, which is intended to study changes in key operational areas such as self-response and in-field enumeration, among others. It will focus on hard-to-count and historically undercounted populations, with the Census Bureau selecting certain test sites to facilitate this research. A 2028 "Census Dress Rehearsal" is planned as a dry run of enumeration operations. The Census Bureau expects this to have a similar scope to the 2018 End-to-End Census Test, which had the objective of testing various key operational components of the 2020 census, such as new technologies, data collection methods, and outreach.

Congressional Oversight

Throughout the decade leading up to and following the decennial census, Congress has typically provided oversight on census operations. Two examples of potential oversight are discussed below.

Operations Oversight

Leading up to a decennial census, Congress has typically provided oversight on how the Census Bureau uses its annual appropriations on census operations. Costs for the decennial census ramp up as the date of enumeration draws nearer. Congress has historically used appropriations report language as an avenue to direct the Census Bureau to report on certain operations. Required reporting has ranged from specific operations, such as the use of privacy protection technologies, to monthly status reports on general decennial census operations. Planned innovations for the 2030 census may impact operations and the decennial census data itself, as some previously introduced innovations faced challenges in implementation. Congress may consider remaining involved in understanding and providing oversight as the Census Bureau potentially further develops these operations for 2030.

Data Quality and Confidentiality

The trade-offs between data quality and privacy are an ongoing feature of discussions on federal statistics, including those from the decennial census.

Members of Congress may consider maintaining oversight of the Census Bureau's plans and implementation of privacy protection for the decennial census along with the level of trade-offs for data quality. This could include a focus on the disclosure avoidance methods the Census Bureau uses and how they may have changed from 2020.