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4 A Guidebook for Using American Community Survey Data for Transportation Planning differences between ACS and the traditional census Long Form data collection, as these differ- ences will have direct impacts on data users, such as transportation planners. Section 3 describes the ACS data products and how users can access these products and other information related to ACS through the Census Bureau's American FactFinder website. This sec- tion provides lists of available tables and examples of the different types of ACS tables that are available to users. Section 4 includes several sections that summarize the special challenges of using ACS data. This section begins by summarizing research on ACS data quality. The first subsection attempts to inform data users about the accuracy and potential biases in the ACS estimates. Next addressed are two aspects of ACS that may be new to traditional Census data users--the need for the accumulation of data across geographic areas and over time and the need to con- sider the effects of data disclosure limitations in designing analyses. The ways in which these issues should affect data users' choices about data analysis strategies are demonstrated. Section 4 then turns to less strategic, and more hands-on, ACS data use issues. Described is the importance of measuring, understanding, and reporting the precision levels in ACS esti- mates. There is discussion of the comparison of ACS estimates to Census 2000 results, and finally, discussion of effective ways that analysts can take advantage of having more frequent estimates of census variables. Sections 5 through 9 provide case study examples of potential transportation planning analyses using ACS data. Section 5 discusses the most common uses of census data--descriptive analyses and policy planning analyses. Section 6 describes the application of ACS to trend analyses. With ACS's annual data releases, the opportunities for performing more interesting trend analyses increase. Section 7 provides case studies on transportation market analyses, including environmental justice analyses. Section 8 discusses the use of ACS in the design and analysis of transportation surveys, particularly household travel surveys. Finally, Section 9 describes how ACS data can be used in place of decennial census data in travel demand modeling analyses. 1.4 Additional Information Sources for an Introduction to ACS The Census Bureau and others have developed several documents that provide an introduc- tion to ACS. The Census Bureau American Community Survey website (www.census.gov/acs/www/index. html) includes several introductory documents in Portable Document Format (pdf) in the Survey Basics section, including American Community Survey: A Handbook for State and Local Officials (issued December 2004); Congressional Toolkit (issued Spring 2004), which includes several documents that "explain how and why the survey is conducted, its benefits, and how to obtain additional information." The documents, which are provided on CD-ROM to congressional staff, include ACS Tool Kit, an introductory summary of ACS; ACS Housing Fact Sheets, a summary of housing information collected in the ACS and justification for why this information is collected; ACS Population Fact Sheets, a summary of population information collected in the ACS and justification for why this information is collected;

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Introduction 5 ACS Questionnaire, a copy of the mailed questionnaire; ACS Instruction Guide, a copy of the instructions that accompany the questionnaire; and Questions and Answers, an introduction to ACS presented in a question-and-answer format. ACS News Media Toolkit (updated October 2005), which includes a number of documents that summarize ACS for media data users. In addition to the Census Bureau overviews, other researchers and data users have assembled many good descriptions of ACS that are suitable for interested new potential users. We recom- mend the Population Reference Bureau's September 2005 issue of its Population Bulletin, which can be found at www.prb.org/pdf05/60.3The_American_Community.pdf. Transportation users of ACS can find summaries of many relevant ACS issues in FHWA's Census Transportation Planning Package (CTPP) status reports, which can be found at www.fhwa.dot.gov/ctpp/status.htm. Finally, motivated prospective ACS users could benefit greatly from the on-line course on ACS offered by Statistics.com ($399 as of January 2006) and can be found at www.statistics.com/ content/courses/census/index.html.