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Using the American Community Survey: Benefits and Challenges
3-F.1 Using ACS Profiles and Rankings
ACS products that are likely to be of broad general interest include single-year profiles, providing key 1-year period estimates for governmental and statistical areas with at least 65,000 people; multiyear profiles, providing the same key 1-year period estimates for the current year and four prior years for areas with at least 65,000 people; and single-year ranking tables and charts comparing states and large cities on selected 1-year period estimates.16 These products will be timely and easy to reference. They will be the starting point for press releases from government officials and media articles describing what has occurred in a city, county, metropolitan area, or state since the year before and in comparison with other areas.
For trend analysis using multiyear profiles, public officials and the media must take care to avoid making too much of year-to-year differences that are within the margin of error (see Section 3-C.1.b). Just as the media have educated the public about the margin of error in public opinion polls, so should they take on the responsibility to educate readers about the margin of error from ACS estimates in profiles and other data products. The Census Bureau will provide margins of error for estimates in single-year profiles. In multiyear profiles, it will indicate estimates for each year that are statistically significantly different from the estimates for the current year.17
Similarly, for comparisons across areas using 1-year period estimates, public officials, the media, and readers must learn that, in most cases, the difference between, say, the city with the highest school-age poverty rate and the city with the next highest rate is not necessarily indicative of a real difference or even of the real ordering. In fact, the estimates for 5 or 10 of the cities with the highest rates may be not be statistically different, so that it is appropriate to say only that City A falls into the top, middle, or bottom group of cities rather than to assign it an individual rank-order number. Moreover, when the subsequent year’s period estimates are released, and City A has moved, for example, from number 1 to number 2, 3, 4, or 5 in school-age poverty, the reader should not conclude that school-age poverty has necessarily declined in City A relative to the other cities on the basis of one year’s difference in rankings.
Although sampling error affects such uses of the ACS data as trend analysis and comparative rankings, the regularly updated ACS estimates will be more helpful to users than the once-a-decade estimates from the
Multiyear profiles will be published for geographic areas defined according to the latest known boundaries for all years shown.
The Census Bureau provides 90 percent margins of error; for agreement with standard statistical practice, it should provide 95 percent margins of error instead (refer back to Box 2-5).