Before the first stage of address sampling can proceed for each years main sampling, each block must be assigned to one of the five sampling strata. The ACS produces estimates for geographic areas having a wide range of population sizes. To ensure that the estimates for these areas have the desired level of reliability, areas with smaller populations must be sampled at higher rates relative to those areas with larger populations. To accomplish this, each block and its constituent addresses are assigned to one of five sampling strata, each with a unique sampling rate. The stratum assignment for a block is based on information about the set of geographic entities-referred to as sampling entities- which contain the block, or on information about the size of the census tract that the block is located in, as discussed below. Sampling entities are defined as:
- Places with active and functioning governments.5
- School districts.
- American Indian Areas/Alaska Native Areas/Hawaiian Home Lands (AIANHH).
- American Indian Tribal Subdivisions with active and functioning governments.
- Minor civil divisions (MCDs) with active and functioning governments in 12 states.6
- Census designated places (in Hawaii only).
The sampling stratum for most blocks is based on the measure of size (MOS) for the smallest sampling entity to which any part of the block belongs. To calculate the MOS for a sampling entity, block-level counts of addresses are derived from the main MAF. This count is converted to an estimated number of occupied HUs by multiplying it by the proportion of HUs in the block that were occupied in Census 2000. For American Indian and Alaska Native Statistical Areas (AIANSA7
) and Tribal Subdivisions, the estimated number of occupied HUs is also multiplied by the proportion of its population that responded as American Indian or Alaska Native (either alone or in combination) in Census 2000. For each sampling entity, the estimate is summed across all blocks in the entity and is referred to as the MOS for the entity. In AIANSAs if the sum of these estimates across all blocks is nonzero, then this sum becomes the MOS for the AIANSA. If it is zero (due to a zero census count of American Indians or Alaska Natives), the occupied HU estimate for the AIANSA is the MOS for the AIANSA (see Hefter, 2006a, for additional details). Each block is then assigned the smallest MOS of all the sampling entities in which the block is contained and is referred to as Smallest Entity Measure of Size, or SEMOS.
If the SEMOS is greater than or equal to 1,200, the stratum assignment for the block is based on the MOS for the census tract that contains it. The MOS for each tract (TMOS) is obtained by summing the estimated number of occupied HUs across all of its blocks. Using SEMOS and TMOS, blocks are assigned to the five strata as defined in Table 4.1 below. These strata are consistent with the sampling categories used in Census 2000 except for the category for sampling entities with MOS less than 800, which has been split into two categories for ACS.
Functioning governments have elected officials who can provide services and raise revenue.
The 12 states are considered "strong" MCD states and are: Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Wisconsin.
AINSA is a general term used to describe American Indian and Alaska Native Village statistical areas. For detailed technical information on the Census Bureau's American Indian and Alaska Native Areas Geographic Program for Census 2000, see Federal Register Notice Vol. 65, No. 121, June 22, 2000.
Table 4.1 Sampling Strata Thresholds for the ACS/PRCS
||Smallest Entity Measure of Size (SEMOS) and Tract Measure of Size (TMOS)
|Blocks in large sampling entities (SEMOS >1,200) and large tracts
|Blocks in large sampling entities (SEMOS >1,200) and small tracts
|Blocks in small sampling entities
||800 ≤SEMOS ≤1,200
|Blocks in smaller sampling entities
||200 ≤SEMOS <800
|Blocks in smallest sampling entities
||SEMOS < 200
The figure shows a census block that is in City A and is also contained in School District 1. Therefore, it is contained wholly in three sampling entities:
- County (not shown).
- Place with active and functioning government-City A.
- School district.
FIGURE 4.2: ASSIGNMENT OF BLOCKS (AND THEIR ADDRESSES) TO SECOND-STAGE SAMPLING STRATA
(Note that the land area of a sampling entity does not necessarily correlate to its MOS)
Suppose the MOS for City A is 600 and the MOS for School District 1 is 1,100. Then the SEMOS for the census block is 600 and it is placed in the 200 ≤SEMOS ≤800 stratum.
Suppose the MOS for City A is 1,300 and the MOS for School District 1 is 1,400, then the SEMOS for the block is 1,300. Since the SEMOS for the block is greater than 1,200, the block will be assigned to one of the two strata with SEMOS >1,200 depending on the size of the census tract (TMOS-not shown in the diagram). In this example, suppose the TMOS is 1,800, then the census block will be placed in the 1,200