population in 1990 in comparison with 1980. At the same time, there was increasing interest in obtaining more frequent population estimates for small areas.
To counter this problem of declining long-form response rates and to provide more frequent data for small areas, in 1994 the Census Bureau decided to move toward a continuous measurement design similar to one that had been proposed years earlier by Leslie Kish (see National Research Council, 1995, p. 71). This continuous measurement survey was named the American Community Survey, and the Census Bureau set a goal of conducting a short-form-only census in 2010 and to fully implement the ACS by then. It was expected that the ACS could provide estimates for small areas that were about as precise as long-form-sample estimates for small areas by accumulating samples over 5 years. However, very early in the development process, rising costs led to a decision to scale back the originally planned size of 500,000 housing units per month to a sample of 250,000 housing units per month (National Research Council, 1995, p. 127). This decision to reduce the desired sample size had a significant deleterious effect on the ability of the ACS to provide reliable 1-year data for small areas.
Each month, the ACS questionnaire—which is similar in content to the old census long form—is mailed to 250,000 housing units across the nation. The units have been sampled from the Census Bureau’s Master Address File using a probability sample design in which housing units in small areas are oversampled. As with the long form of the census, response to the ACS is required by law.
The ACS mail questionnaire uses a matrix layout for questions on sex, age, race, ethnicity, and household relationship. It provides space for information on five household members; information on additional household members is gathered through a follow-up telephone survey. The ACS instructs the household respondent to provide data on all people who, at the time of completing the questionnaire, have been living or staying at the household address for more than 2 months (including usual residents who are away for less than 2 months). Individuals in the ACS samples that reside in group quarters (such as college dormitories and prisons) are counted at the group quarters location, in effect applying a de facto residence rule regardless of how long an individual has lived or expects to live in the group quarters.
The residential housing unit addresses in the ACS sample with usable mailing addresses—about 95 percent of each month’s sample of 250,000 addresses—are sent a notification letter 4 days before they receive a questionnaire booklet, and a reminder postcard is sent 3 days after the questionnaire mailing. Whenever a questionnaire is not returned by mail within 3 weeks, a second questionnaire is mailed to the address. If there is no response to the second mailing, and if the Census Bureau is able to obtain a telephone number for the address, trained interviewers conduct telephone interviews using computer-assisted telephone interviewing (CATI) software.