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University Library, University of Illinois at Urbana-Champaign

US Census

Find information about the Decennial Census and the American Community Survey for students and researchers.

About the American Community Survey

The American Community Survey (ACS) is an ongoing survey conducted by the U.S. Census Bureau since 2005. In 2010, the ACS replaced the decennial census long form as the source of sample data for population and housing indicators. The short form of the decennial census is still distributed to obtain a count of the entire U.S. population.

ACS asks questions about age, sex, race, family and relationships, income and benefits, health insurance, education, veteran status, disabilities, employment location and mode of travel to work, place of residence, price paid for living essentials. You can review the annual questionnaires to see the precise questions asked each year. For side-by-side comparisons of ACS questionnaires, visit the ACS Questionnaire Changes page

There are both benefits and challenges associated with replacing the long form with ACS. The primary benefit is the freshness of data: ACS data is collected every year, whereas the decennial census was collected every ten years. Challenges include a higher sampling error for the ACS, due to a smaller sample population, and difficulties comparing data from year to year.

ACS Estimates

Each year, the Census Bureau releases 1-, 3- and 5-year estimates based on information gathered in the ACS. Important note: if you wish to compare ACS estimates to earlier Decennial Census data, please review these cautions first.

In deciding which estimate you want to use, you should consider the currency of data; the geographic size of your population; and the acceptable sample size/reliability of the data. The Census Bureau chart below shows distinguishing features of the different estimates.

1-year estimates

3-year estimates

5-year estimates

12 months of collected data

36 months of collected data

60 months of collected data

Data for areas with populations of 65,000+

Data for areas with populations of 20,000+

Data for all areas

Smallest sample size

Larger sample size than 1-year

Largest sample size

Less reliable than 3-year or 5-year

More reliable than 1-year; less reliable than 5-year

Most reliable

Most current data

Less current than 1-year estimates; more current than 5-year

Least current

Best used when

Best used when

Best used when

Currency is more important than precision

Analyzing large populations

More precise than 1-year, more current than 5-year

Analyzing smaller populations

Examining smaller geographies because 1-year estimates are not available

*Note: 3-year estimates were discontinued in 2015 due to budget cuts. 

Precision is more important than currency

Analyzing very small populations

Examining tracts and other smaller geographies because 1-year estimates are not available


This guide is adapted from UC San Diego's Census Research Guide by Kelly L. Smith.