Over 3.7 million block clusters were formed that covered the entire United States, except remote Alaska.² Each cluster included one census collection block or a group of geographically contiguous blocks, in which the block(s) were expected to be enumerated using the same procedure (e.g., mailout/mailback) and to contain, on average, about 30 housing units on the basis of housing unit counts from an early version of the 2000 Master Address File (MAF). The average cluster size was 1.9 blocks.

Next, clusters were grouped into four sampling strata: small (0–2 housing units), medium (3–79 housing units), large (80 or more housing units), and American Indian reservations (in states with sufficient numbers of American Indians living on reservations). Systematic samples of block clusters were selected from each stratum using equal probabilities, yielding about 29,000 block clusters containing about 2 million housing units, which were then visited by Census Bureau field staff to develop address lists.

The sample at this stage was considerably larger than that needed for the A.C.E. The reason was that the Census Bureau had originally planned to field a P-sample of 750,000 housing units for use in ICM, and there was not time to develop a separate design for the planned A.C.E. size of about 300,000 housing units. So the ICM block cluster sample design was implemented first and then block clusters were subsampled for A.C.E., making use of updated information from the address listing about housing unit counts.³

After completion of the address listing and an update of the MAF, the number of medium and large block clusters was reduced, using differential sampling rates within each state. Specifically, medium and large clusters classified as minority on the basis of 1990 data were oversampled to improve the precision of the DSE estimates for minority groups. Also, clusters with large differences in housing unit counts from the P-sample address list and the January 2000 version of the MAF were oversampled in order to minimize their effect on the variance of the DSE estimates.

The next step was to stratify small block clusters by size, based on the current version of the MAF, and sample them systematically with equal probability