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CHAPTER 6 Trend Analyses Using ACS Data This section describes trend analysis as a common application of census data. It is organized as follows. Section 6.1 defines trend analysis and its uses, and provides some examples of how analysts use census data for these purposes. A detailed list of specific examples of uses of census data for this purpose is provided at the end of the section. Section 6.2 describes some benefits and limitations of shifting from census to ACS data related to trend analysis. Finally, Section 6.3 is a case study showing how to do trend analysis using ACS data. 6.1 Trend Analysis Trend analysis is a common application of census data that consists of examining the change in a given characteristic over time to understand future needs and guide policy planning. Trend analysis requires the availability of data for multiple time periods and consistency of the question and response categories over the time period considered. The decennial census, with its standard questions, relatively consistent data collection procedures, and comprehensive coverage, has been a good tool for performing trend analysis. Transportation planners have often used decennial census data to consistently monitor changes in population growth rates, population composition, and work trip behavior in a given area. Trend analysis is normally performed to Evaluate the overall pattern of change (magnitude and direction) in a characteristic over time; Compare the levels of a characteristic between particular time periods, such as before and after an event; Compare geographic areas across time; Compare different populations across time to evaluate absolute and relative changes in a characteristic; and Examine data over time to help in forecasting future conditions. 6.1.1 Examples of Use Census data have been used by transportation planners and others for analyzing a variety of demographic and journey to work trends. Figures 6.1 and 6.2 show examples of how trend analyses using decennial census data have been presented. Figure 6.1 shows a national trend of zero-vehicle households,51 and Figure 6.2 shows a national trend of means of transportation to work.52 A list of specific examples of using census data to do trend analysis also is provided at the end of this section. 51 Murakami, E., 2003, "Households without Vehicles, 2000." See 52 N. Srinivasan, 2002, "Journey to Work Trends." See 102

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Trend Analyses Using ACS Data 103 Percent 50 Black / African American Households 45 All Hispanic Households 40 All Households 35 30 25 20 15 10 5 0 1970 1980 1990 2000 Year Figure 6.1. National trend of zero-vehicle households. 6.1.2 ACS for Trend Analysis Because of the frequency of data releases, ACS has the potential to greatly improve the trend analyses that transportation planners can conduct. While the decennial census Long Form data provide the opportunity to analyze trends at 10-year intervals, ACS will allow for the analysis of trends for intervals as short as 1 year. Percent 100 90 80 70 60 50 40 30 20 10 0 1960 1970 1980 1990 2000 Year Worked At Home Other Means Walked Public Transportation Private Vehicle Figure 6.2. National trend of means of transportation to work.