The American Community Survey (ACS) is the cornerstone of the U.S. Census Bureau's effort to keep pace with the nations ever-increasing demands for timely and relevant data about population and housing characteristics. The new survey provides current demographic, social, economic, and housing information about America's communities every year-information that until now was only available once a decade. Implementation of the ACS is viewed by many as the single most important change in the way detailed decennial census information is collected since 1940, when the Census Bureau introduced statistical sampling as a way to collect "long-form" data from a sample of households.

The ACS and the reengineering of the decennial census will affect data users and the public for decades to come. Beginning with the surveys full implementation in 2005, the ACS has replaced the census long-form questionnaire that was sent to about one-in-six addresses in Census 2000. As with the long form, information from the ACS will be used to administer federal and state programs and distribute more than $300 billion a year in federal funds. Obtaining more current data throughout the decade from the ACS will have long-lasting value for policy and decision-making across federal, state, local, and tribal governments, the private sector, and virtually every local community in the nation.

In 1994, the Census Bureau started developing what became the ACS with the idea of continuously measuring the characteristics of population and housing, instead of collecting the data only once a decade with each decennial census. Testing started in four counties across the country and with encouraging results, the testing expanded to 31 test sites by 1999. Realizing that a continuous program would also be collecting information during a decennial census, the sample was increased to about 800,000 addresses in 2000 and continued its demonstration period through 2004. This was a national sample that yielded results for the country, states, and most geographic areas with 250,000 or more population.

Comparing the 2000 ACS data with the results from the Census 2000 long form proved that the idea of a monthly survey was feasible and would generate quality data. With some changes to the sample design and other methodologies, the ACS was fully implemented in 2005 with a sample of three million addresses each year. A sample also was implemented in Puerto Rico, where the survey is known as the Puerto Rico Community Survey (PRCS). In 2006, a sample of group quarters facilities was included so that estimates from the ACS and the PRCS would reflect complete characteristics of all community residents.

Currently, the ACS publishes single year data for all areas with populations of 65,000 or more. Among the roughly 7,000 areas that meet this threshold are all states, all congressional districts, more than 700 counties, and more than 500 places. Areas with populations less than 65,000 will require the use of multiyear estimates to reach an appropriate sample size for data publication. In 2008, the Census Bureau will begin releasing 3-year estimates for areas with populations greater than 20,000. And, we plan to release the first 5-year estimates for all census tracts and block groups starting in 2010. These multiyear estimates will be updated annually, with data published for the largest areas in both 1-, 3-, and 5-year formats, and for those meeting the 3-year threshold in both 3- and 5-year formats. Of course, even the smallest communities will be able to obtain ACS data based on 5-year estimates annually.

This ACS Design and Methodology Report is an update of the first unedited version that was released in 2006. We released that draft version because of the need to provide data users with information about the first full sample year of the survey. The version released in 2006 provided design and methodology information for the 2005 ACS only.

This version of the ACS Design and Methodology Report includes updated information reflecting survey changes, modifications, and improvements through the end of 2007. Many portions of each chapter have been revised. We hope that data users find this report helpful and that it will aid in improving the public's understanding of the ACS statistical design and the methods it uses.

The ACS program has been successful in large part because of the innovation and dedication of many people who have worked so hard to bring it to this point in time. With this publication of the ACS Design and Methodology Report , many individuals-both past and current-deserve special congratulations. From those early beginnings with a handful of designers, survey methodologists, and technical experts, through full implementation, countless individuals have contributed to the surveys successful implementation.

All of the primary survey activities are designed and managed by the staff at Census Bureau headquarters in Suitland, MD, who continually strive to improve the accuracy of the ACS estimates, streamline its operations, analyze its data, conduct important research and evaluation to achieve greater efficiencies and effectiveness, and serve as educational resources and experts for the countless data users who come to the Census Bureau in need of technical assistance and help. In addition, the Census Bureau's field partners provide many of the critical day-to-day activities that are the hub of the ACS existence. The ACS, which is the largest household survey conducted by the federal government, could not be accomplished without the dedication and effort of staff at the Census Bureau's National Processing Center (NPC) in Jeffersonville, IN; the Census Bureau telephone call centers in Jeffersonville, IN; Hagerstown, MD; and Tucson, AZ; and the thousands of field representatives across the country who collect ACS data. In addition, the ACS field operations are run by Census Bureau survey managers in the NPC, telephone call centers and the twelve Regional Offices, all of whom add immeasurably to the smooth and efficient running of a very complex and demanding survey operation.

Finally, the ACS would not have achieved its success without the continued cooperation of millions of Americans who willingly provide the data that are collected each year. The data they provide are invaluable and contribute daily to the surveys exceptional accomplishments. Sincere thanks are extended to each and every respondent who took the time and effort to participate in this worthwhile endeavor.

We invite you to suggest ways in which we can enhance this report in the future. Also, please remember to look for updated versions of this report as the ACS continues in the coming years.