Tip Sheet: U.Va. Cooper Center Demographers Provide Primer on Census Data

REPORTERS: The United States Census Bureau will release American Community Survey one-year data today. 

September 28, 2010 — Census 2010 is over. The snapshot has been taken.

"We are waiting for the photo to develop, and soon we will see how it looks and what it tells us," said Susan Clapp, a statistician in the Demographics and Workforce Group of the University of Virginia's Weldon Cooper Center for Public Service.

In past decennial censuses conducted by the United States Census Bureau, a combination of short and long questionnaires was used to collect data from every American household. Beginning with the 2010 census, only a short form was used. But this does not mean that the Census Bureau is no longer collecting detailed information, Clapp said. The long form, which used to be taken only every 10 years, has been replaced by a new annual survey called the American Community Survey, or ACS.

In the months and years after each decennial census, the Census Bureau releases several datasets showing the results. These data releases and data products can be hard to navigate, Clapp said, "but they are essential in helping us better understand who we are and how we live."

To assist reporters and others with the upcoming data releases, the Cooper Center has compiled the information (below) from the Census Bureau website. On the Cooper Center website, there are tables that further explain what the data reveals.

 
Differences Between the 2010 Census and the American Community Survey

The next few years may be confusing to people who want to understand and use census data because it will come from these two different sources. Understanding the basic differences between the 2010 Census and the American Community Survey is an important first step to clearing up the confusion. The following table shows some notable characteristics of the 2010 Census and the American Community Survey.

  
The American Community Survey is a Survey, Not a Census

The American Community Survey has additional complexities that should be understood. Unlike the Census 2010, which was filled out by everyone, the American Community Survey is filled out by just a sample of people in each community, and their answers are used to generate estimates that describe the entire community population. To make good estimates about the whole population, a sufficient number of people must be surveyed.

For example, the survey responses of one person are not sufficient to make generalizations about even the smallest community in Virginia. Depending on the size of the community, a sample size of 100, 1,000, or even 5,000 people per year may be required.

In large communities, the Census Bureau can sample enough people each year to produce annual estimates. In small communities, however, the annual sample sizes are small and five years' worth of survey data are required to get a reliable base for estimates.
 
American Community Survey Data Covers Different Time Periods

American Community Survey data are combined to provide 12, 36 or 60 months of data. The release of each of these datasets is driven by the population size; one-year estimates are available for large areas (those with population of 65,000 or more), while five-year estimates are available for geographic areas as small as the Census tract (a relatively homogeneous area with an average of 4,000 inhabitants) and block group (an area that is smaller than an entire tract but larger than a single block). It is important to remember that unlike the decennial census, which gives us a snapshot of the population on a single day, the American Community Survey estimates tell us how the population was living over a period of one, three or five years.

Why and How To Use American Community Survey Data

Choosing which American Community Survey dataset to use involves more than simply considering the population size in your area. You must think about the balance between currency and sample size/reliability/precision. The following table shows the features of American Community Survey estimates.

Not Confused enough? Now, Add in the Decennial Census Data

While American Community Survey data have been released each year since 2005, the coming years mark the first time since the survey's inception that new decennial census data will also be released. To further complicate matters, it is also the first time that five-year data from the American Community Survey will be available. To help ease the transition, the following table provides the release dates for the major census products, with 2010 Census release dates shown in bold and American Community Survey release dates shown in italics.

REPORTERS: For further assistance, contact Susan Clapp at susan.clapp@virginia.edu or 434-982-5690.