census long-form sample. From the example above, it is useful information to know what group of cities—top, middle, or bottom—a particular city is part of and to know, year to year, whether that city has remained about the same in relative ranking or, in contrast, has experienced a major change.
Often, estimates will be available not only from the ACS, but also from another data source, and the public, policy officials, and the media will want to know the reasons if the ACS and the other source do not agree. In fact, it is likely that differences will occur between estimates from two data sources because of differences in concepts and definitions, data collection procedures, and other aspects of the two sources.
In addition, users who want to compare 2005 ACS estimates for governmental or statistical areas with 65,000 to 250,000 people with estimates from an earlier period must use a different source—namely, the 2000 long-form sample—as their point of comparison. (The 2005 ACS estimates for areas with 250,000 or more people can be compared with estimates from the Census 2000 Supplementary Survey (C2SS) or any of the 2001–2004 ACS test surveys.) As described in Chapter 2, there are important differences between the long-form sample and the 2005 ACS, involving sample size, population covered, data collection mode, population controls, and others, so that assessment of changes between 2000 and 2005 must be made with great care. In the future, the yearly releases of ACS data will make it possible to assess trends using just the ACS, but the 2000 long-form sample will remain an important comparison source for small areas for some time to come.
With regard to comparisons between the ACS and another source for the same time period, an object lesson is afforded by experience in comparing state estimates of poverty from the CPS ASEC and the ACS supplementary surveys. National estimates from the CPS ASEC are the official poverty estimates for statistical use according to OMB Directive 14. To respond to user needs, the Census Bureau began publishing poverty estimates for states in 1990 from the CPS by averaging 2 and 3 years’ worth of estimates to improve precision. The Census Bureau has also published state poverty estimates from the C2SS and the 2001–2004 ACS test surveys and, now, the 2005 ACS.
Comparisons of trends from the CPS ASEC state poverty estimates averaged over 2 years with those from the C2SS and the ACS 2001–2004 test surveys revealed instances in which the two data sources did not agree on the poverty rate or the direction of change (increase or decrease in poverty). There are many reasons that may explain these differences: