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period of time, instead of providing a snapshot as of a particular point in time, as the census long form did for the April 1 reference date, once every 10 years.

Table 1-1 shows the initial ACS sample sizes and number of completed interviews between 2006 and 2010 (U.S. Census Bureau, 2011a). These sample sizes cumulate to approximately 15.4 million cases (10.4 million completed interviews) over the 5-year period, including the GQ residents.

In an ideal version of the ACS design, data cumulated over 5 years would have comparable estimation reliability to that achieved by recent census long-form samples, even in small areas. Although long-form sample sizes have varied in recent censuses, the last time the long form was administered (as part of the 2000 census), the sample included approximately 18 million housing units, and the data collection resulted in 16.4 million completed questionnaires (National Research Council, 2007). During the first few years of the ACS, the survey was not funded at a level necessary for a comparable sample size, but beginning in June 2011 the target was increased to 3.54 million sampled addresses annually, which, if continued at that level, will bring the ACS housing unit sample size closer to the census 2000 level over 5-year intervals in the future (U.S. Census Bureau, 2011b).

Table 1-2 shows the ACS data release schedule from the survey’s inception through 2013, along with the population thresholds required for each release. Beginning in 2006, the Census Bureau published annual 1-year estimates of characteristics of the U.S. population and housing units for all geographic entities with populations of at least 65,000. Since 2008, 3-year estimates for geographic entities with populations of at least 20,000 have also been reported. The end of 2010 marked a crucial milestone for the ACS, when the first set of estimates based on 5 years of continuous data collection were published for all statistical, legal, and administrative entities, including areas as small as census block groups.

TABLE 1-1 Initial ACS Sample Sizes and Completed Interviews, 2006-2010


Housing Units GQ Residents
Year Initial
Addresses
Selected
Final
Interviews
Initial
Sample
Selected
Final
Interviews

2010 2,899,676 1,917,799 197,045 144,948
2009 2,897,256 1,917,748 198,808 146,716
2008 2,894,711 1,931,955 186,862 145,974
2007 2,886,453 1,937,659 187,012 142,468
2006 2,885,384 1,968,362 189,641 145,311

NOTE: Only a subsample of the housing units that do not respond by either mail or telephone are included in the in-person follow-up, which is the final stage of the ACS nonresponse follow-up effort. More information about response rates is available on the Census Bureau website (http://www.census.gov/acs/www/methodology/response_rates_data/).
SOURCE: U.S. Census Bureau (2011a).


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