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Getting Started with the U.S. Census: American Community Survey v. Decennial Census

Which to Choose--Decennial Census or the American Community Survey? Other Census Programs?

The Census Bureau provides an immense amount of statistics and numeric data though its various programs.  Most research projects will rely on either the Decennial Census or the American Community Survey, however depending on your project, there are several other data programs that could be useful.  If you need statistics and/or data beyond the Decennial Census or American Community Survey, explore the additional tabs on this guide and/or contact the Library.

ACS, Decennial? What are they and what's the difference?

From 1790-2000, the Census collected data every 10 years.  This is the Decennial Census, and from 1790 forward gave researchers a statistical picture of the United States.  However, one of the drawbacks of collecting data in this way is its currency: various factors can cause a geographic area to change dramatically in a short time, and relying on official census statistics that are 3+ years old could pose a problem. Researchers, local governments and others wanted reliable statistics on a more frequent basis.

In 2005, the Census Bureau began gathering data in a new way: the American Community Survey.  This survey is intended to provide the public with a current statistical average of a geographic area over 1-year, 3-year and 5-year time periods (the 3-year ACS ended in 2014 due to budgetary issues).  The time periods correspond to data precision, availability by geography, and currency:

And what happened to the Decennial census?  Its still done and the data is used primarily for congressional redistricting and basic counts of the population (age, race, sex, etc).  Only 10 questions are asked so the amount of data collected is very limited compared with the much more expanded ACS. 

When To Compare ACS Statistics?

If you want to compare one variable across a variety of places, you need to find the survey that includes the smallest size geography you want to compare. For example, if you want to compare the poverty rate in Bayview, CA (population under 2,400) and Richmond, CA (population 104,000), you need to use the ACS that is available for both geographies -- the 5 year estimate.

Very important: DO NOT compare different time periods (such as the 1-year to the 5-year) and DO NOT compare overlapping period (such as the 2014-2018 ACS 5-year estimates to the 2017-2021 ACS 5-year estimate). Click here for more on comparing data.  The Census even provides a Statistical Testing Tool to help with making comparisons.

Questions Asked on the Census

A good strategy for determining if the Census has the statistics you are looking for is examine the questions asked.  There are a couple resources for this information.