information, enabling users to analyze recent data for an area or population group and compare it with other areas and groups.

Moreover, the annual ACS products will enable users to construct time series for analyzing change. To the extent permitted by sampling error, these data will make it possible to detect trends in the percentage of people in an area who are employed, live in poverty, or have attained a college degree, and whether the trends for an area mirror or deviate from national trends. Similarly, the data will show changes in the ethnic makeup of an area, housing costs for homeowners and renters, and many other characteristics of interest to data users. Of course, several factors can interrupt a time series for an area of interest, such as a change in geographic boundaries, a change in the wording of the question used to measure a characteristic of interest, or, occasionally, a revision to the county population estimates that are used to control the ACS estimates.

Data Quality

Another major benefit of the ACS over the census long-form sample should be higher quality of the data in terms of the completeness and accuracy of response. Missing and inaccurate responses are components of nonsampling error that can result in bias in survey estimates, as distinct from the variable error due to the use of a sample (discussed in Section 2-C below). Both kinds of error are important: sampling variability can be so large as to render an unbiased estimate of little use for decision making, while even a very precise estimate in terms of sampling error can be misleading if the bias in the estimate is large (see Box 2-3 for brief descriptions of elements of bias and variability).

The assessment of the likely higher quality of the ACS rests primarily on comparisons of estimates from the Census 2000 Supplementary Survey (C2SS) and the 2000 long-form sample. The C2SS was a full-scale test of the ACS questionnaire and data collection procedures. It included about 587,000 responses from a nationwide sample in 1,203 counties plus samples in 36 counties that were ACS test sites.

By comparing the C2SS and the 2000 long-form sample, the Census Bureau was able to evaluate the relative quality to be expected from the ACS in terms of unit (household) weighted response rates, population coverage, item response rates, and quality control processes. See Box 2-4 for indicators of sample size, household response, population coverage, and item response for each year of the ACS.5 The C2SS equaled or outperformed


The indicators can be accessed from the main ACS web site ( under “Using the Data” or by selecting a subject area for which ACS data are desired, clicking on “survey methodology” and then on “quality measures”:

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