(April 2009) While the main function of the U.S. decennial census is to provide counts of people for the purpose of Congressional apportionment, the primary purpose of the ACS is to measure the changing social and economic characteristics of the U.S. population. As a result, the ACS does not provide official counts of the population in between censuses. Instead, the Census Bureau's Population Estimates program will continue to be the official source for annual population totals, by age, race, Hispanic origin, and gender. ACS estimates are therefore controlled to match the Census Bureau's annual population estimates, by age, sex, race, and Hispanic origin. For more information about population estimates, visit the Census Bureau's website at www.census.gov/popest/estimates.php.

Although the questions used in the ACS are very similar to those included on the long form used in the 2000 decennial census, there are some important differences between the two surveys. While the decennial census has provided a snapshot of the U.S. population once every 10 years, the ACS has been described as a "moving video image, continually updated to provide much needed data about our nation in today's fast-moving world."1

Because ACS data are collected continuously, they are not always comparable to data collected from the decennial census. For example, in the case of employment statistics, both surveys ask about employment status during the week prior to the survey. However, data from the decennial census are typically collected between March and August, whereas data from the ACS are collected each month and reflect employment throughout the year. Differences in these responses may in turn affect data on commuting, occupation, and industry.2

Finally, in 2006, for the first time, the ACS included the population living in group quarters (such as jails, college dormitories, and nursing homes). As a result, 2006 and later ACS data may not be comparable with data from earlier ACS surveys. This is especially true for estimates of young adults and the elderly, who are more likely than other groups to be living in group quarters facilities.


  1. Kathleen B. Cooper, "Halfway to the 2010 Census: The Countdown and Components to a Successful Decennial Census," paper presented to the House Subcommittee on Federalism and the Census, U.S. House of Representatives, Washington, DC, April 19, 2005.
  2. U.S. Census Bureau, Meeting 21st Century Demographic Data Needs—Implementing the American Community Survey: Report 5: Comparing Economic Characteristics With Census 2000 (2004), accessed online at www.census.gov/acs/www/Downloads/Report05.pdf, on Feb. 28, 2008.