structure” includes the MAF or comprehensive list of housing units in the United States; the mailout of forms to most households; the follow-up of nonrespondents and the associated local enumeration office structure; and data capture, editing, imputation, and preparation of data products. Under some assumptions, the marginal cost of the long-form sample was primarily that of the additional data capture and processing; hence, the cost savings through use of the paired strategy. Another source of cost savings is that, with the paired strategy, the MAF needed to be updated only once a decade, whereas the ACS requires the MAF to be constantly updated. An ongoing survey such as the ACS requires separate continuous data collection and processing, as well as continuous estimation, tabulation, and publication operations. Moreover, a continuous survey is never likely to obtain as high a mail response rate as the long form (which benefited from the massive publicity surrounding the decennial census) and hence requires more costly field follow-up.
From the user perspective, a major disadvantage of the paired strategy was that the data collected became less relevant and more out of date the farther one was from Census Day. Typically, because of the need to give priority to the head count processing, the long-form-sample data were not released until 2 years or more after Census Day, and they were not updated for another 10 years. How much error resulted from the use of out-of-date information depended on how quickly the population of an area changed over time.
For example, consider estimates of the immigrant (foreign-born) population for the cities of Los Angeles, San Diego, and San Francisco, California, provided by the ACS test surveys conducted in 2000–2004. The data show complex patterns of stability and change within and among the three cities across this short time span—patterns that would be difficult to predict if the only data available were from a long-form sample every 10 years. The percentages of immigrants in the population of all three cities did not change between 2000 and 2004, but the percentages of the immigrant population who arrived after 2000 and who became citizens grew significantly. In Los Angeles, the percentage of people aged 5 and older who spoke a language other than English increased significantly between 2000 and 2004, but in San Diego and San Francisco, this percentage did not change.2
The effects of Hurricanes Katrina and Rita on the populations of New
At http://factfinder.census.gov, specify “get data” for the ACS and go to “Data Profiles, Selected Social Characteristics” for a specific year. All web addresses are current as of March 2007.