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2 A Guidebook for Using American Community Survey Data for Transportation Planning of population sizes, population characteristics, population growth levels, and difficulty of enumeration. The data collection effort for the 31 test sites has been performed annually since 1999. In addition to the test site program, the Census Bureau performed a large-scale (1,203 counties) operational test of ACS methods in the year 2000, entitled the Census 2000 Supple- mentary Survey (C2SS). In 2002, 2003, and 2004, the data for the 31 test sites were supplemented with data collected for the 1,203 counties in the C2SS. The Census Bureau began full implementation of ACS for all housing units in 2005. Begin- ning in 2006, the Census Bureau will start collecting group quarters data, as well as housing unit data. Thus, from 2006 onward, ACS should provide comparable coverage to the decennial census Long Form data collection. 1.2 Some Important Implications of ACS for Data Users The discontinuation of the decennial census Long Form and the implementation of ACS will significantly affect how transportation planners access, use, and interpret data on population and household characteristics. Later sections of this guidebook will discuss the details of ACS implementation and how ACS will affect transportation data users' analyses. Some of the more important implications of ACS for transportation data users are summarized here. These, and other ACS issues, are developed further later in the guidebook. 1.2.1 Frequency of Data Releases The primary benefit of ACS is that the data are being collected and will be disseminated more frequently than the once-in-10-years decennial census Long Form data. Data users will no longer need to rely on aging "snapshot" estimates of population and housing characteristics. Instead, they will be able to use more recently collected data whose accuracy and relevance will not depend on how closely the analysis year conforms to the decennial census year. In addition, the increased frequency of data releases will enable data users to analyze trends over shorter time periods. 1.2.2 Differences between Census 2000 and ACS Unfortunately, the ACS's differences with the previous census Long Form, in terms of data collection procedures and questions, will make it more difficult for users to compare ACS results with previous census estimates and to understand longer-term trends in demographic, socioe- conomic, and economic characteristics. Determining whether differences between ACS estimates and year 2000 Census estimates reflect actual differences in the populations of interest will require analysts to understand the survey differences and to be able to perform significance tests on sample data. 1.2.3 Reduced Sample Size and the Need for Data Accumulation For the next few years, the ACS sample size will be equivalent to slightly less than 1-in-40 addresses. The decennial census Long Form was sent to about 1-in-6 addresses, with some areas with slightly higher rates and some with slightly lower rates. Because of the reduced sample size of ACS, it is not possible to replicate the decennial census Long Form data on an annual basis. For most analyses, data users will need to rely on estimates derived from accumulating ACS data across years. ACS annual estimates will be released for areas