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U.S. Census Research Guide

The U.S. Census Bureau conducts several data collection programs about the U.S. population which have their own language, geography, and data portals. This guide is based on a similar guide created by Kelly Smith at UCSD.

What is the Decennial Census?

What is the Decennial Census?

A census is a complete count of a population, generally a nation, conducted by government, as of a fixed date."

International Encyclopedia of the Social & Behavioral Sciences (2nd Edition), 2015, Page 302

The United States Constitution (Article 1, Section 2) mandates a decennial census for the purpose of Congressional apportionment. The first Census was conducted in 1790 and counted only heads of household. Over time the Census has expanded to count every person in the U.S. including information on their age, race, and ethnicity as well as a variety of social and economic characteristics.  While there are undoubtedly portions of the population that go uncounted, it is the closest we have to complete demographic and economic data on the U.S. population.

Census Questions & Variables

Census Questions & Variables

Questions change from Census to Census (sometimes dramatically), which means that the statistics available change from decade to decade. The easiest way to find out what information is available for a specific Census is to look at the Census questionnaires. If the question wasn’t asked, the information isn’t available. It’s that simple. For copies of the original Census Questionnaires since 1790, see the Census publication Measuring America: The Decennial Censuses from 1790 to 2000. It is also available in print at the Library.

Between 1970 and 2000 the Decennial Census included a “short form” with questions answered by every household in the country (a census), as well as a “long form” that is answered by about 1 in 6 households (a sample). The Census Bureau began using sampling in 1940 and experimented with various methods in the 1940, 1950, and 1960 Censuses.  Questions on the short form (age, race, etc.) are the basis for the Census 100% data (available in Summary file 1 & 2). Questions on the long form (education, income, housing, etc.) are the basis for the Census sample data (available in summary file 3 & 4). The 2010 Census did not include a long form; instead, sample data previously collected on the long form is now collected regularly through the American Community Survey (ACS).

Starting Points for Census Statistics

Other Censuses

2020 Census Data Releases

2016-2020 American Community Survey (ACS) 5-year estimates now available

  • Released March 17, 2022, the 2016-2020 American Community Survey (ACS) five-year estimates, featuring more than 40 social, economic, housing and demographic topics such as home ownership rates and costs, health insurance coverage and educational attainment are now available at

Estimates of Undercount and Overcount in the 2020 Census (March 10, 2022)

2020 Census Redistricting Results, via

2020 Census Redistricting Results, Release August 12, 2021

2020 Census Redistricting Data hosted at NHGIS

  • The National Historic GIS repository also provides 2020 GIS boundary files, geographic crosswalks between 2010 and 2020 census blocks, and demonstration data to assess 2020 data quality due to changes in privacy protections implemented by the Census Bureau.

2020 Census Redistricting Data by State - CISER @ Cornell

  • On August 12, 2021, the Census Bureau released Public Law 94-171 data, or Redistricting data, in four parts per state. CISER @ Cornell has integrated these and made them available in convenient ready-to-use formats -- SAS, SPSS, STATA, and CSV.  CISER also provided program files in SAS, SPSS, and Stata to read the CSVs.

2020 Census Apportionment Results, Released April 26, 2021

A Preliminary Analysis of U.S. and State Level Results From The 2020 Census

Computing Apportionment