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geographic areas that are small in population size. In approximately 4.5 percent of places1 in the nation, over 10 percent of the total population resides in group quarters, and 1.3 percent of places have over 25 percent of their population in group quarters. The lack of accurate data about the GQ population can adversely affect the ACS estimates produced for such places and, generally, for small geographic areas, especially because GQ residents tend to be systematically different from the household population in the communities where they live. Moreover, the fact that many geographic areas have none of their GQ facilities included in the ACS sample can substantially alter the characteristics of the total population year by year, even in small communities where GQ facilities represent only a small proportion of the total population.

The U.S. Census Bureau requested the Committee on National Statistics at the National Academy of Sciences/National Research Council to convene a panel to conduct an in-depth review of the statistical methodology for measuring the GQ population in the ACS. The panel was to consider user needs for ACS data on various components of the GQ population and, in light of user needs and considerations of operational feasibility and compatibility with the treatment of the household population in the ACS, recommend alternatives to the current sample design, weighting procedures, and other methodological features that can make the GQ data from the ACS more useful for users of small-area data.

THE PROBLEM

Difficulties associated with measuring the GQ population are not limited to the ACS. The accurate classification and enumeration of the GQ population has also been an ongoing concern for the decennial census (National Research Council, 2004). The operational challenges associated with collecting data from nonhousehold populations are similar in the ACS and the census. However, the fact that the ACS must rely on a sample of what is a small and very diverse population, combined with limited funding available for survey operations, makes the ACS GQ sampling, data collection, weighting, and estimation procedures more complex and the estimates more susceptible to problems stemming from these limitations. The concerns are magnified in small areas, particularly in terms of detrimental effects on the total population estimates produced for small areas. The reasons for this are among the main topics of this report.

One of the methodological features that adversely affect ACS estimates for a large number of small geographic areas is that GQ populations are sampled at the state level, without controlling for their distribution at substate levels of

1The Census Bureau defines a “place” as a concentration of population either legally bounded as an incorporated place, such as a city or town, or delineated for statistical purposes as a census-designated place.



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