The Census Bureau employs a two-stage sample design to select the SIPP sample. The two stages are (1) selection of primary sampling units (PSUs) and (2) selection of address units within sample PSUs. Census Bureau interviewers follow an established procedure to identify sample members within the selected address units.
The frame for the selection of sample PSUs consists of a listing of U.S. counties and independent cities, along with population counts and other data for those units from the most recent census of population. Counties either are grouped with adjacent counties to form PSUs or constitute a PSU by themselves.
Following the formation of the PSUs, the smaller ones, called non-self-representing (NSR) PSUs, are then grouped with similar PSUs in the same region (South, Northeast, Midwest, West) to form strata; census data for a variety of demographic and socioeconomic variables are used to determine the optimum groupings. A sample of NSR PSUs is selected in each stratum to represent all PSUs in the stratum. All of the larger PSUs are included in the sample and are called self-representing (SR) PSUs.
SIPP selects addresses from five separate, non-overlapping sampling frames maintained by the Census Bureau. They are unit (formerly called the address enumeration districts [Eds] frame); area (area EDs frame); group quarters (special places frame); housing unit coverage; a coverage improvement frame, and a new-construction (or permit) frame. The first three frames are based on census counts from the most recent decennial census; unit and area frames are determined by a process called .address screening,. which has been done at the block level since 1990. The unit frame lists addresses of housing units located in census blocks in areas that issue building permits and in which at least 96 percent of the addresses are complete (with street name and house number). The area frame contains addresses from the remaining census blocks that are not in permit-issuing areas, or where more than 4 percent of the addresses in the blocks are missing. Those addresses are mostly in rural areas. The group quarters frame includes boarding houses, hotel rooms, and institutions that are found in the decennial census but are not counted as housing units. Together, the three frames provide almost 90 percent of the sample addresses for each SIPP panel.
The coverage improvement frame is used to include addresses of housing units that were missed in the census count but were found in postenumeration surveys. The percentage of sample addresses from this frame is typically small (0.1 percent of the sample addresses in the 1986 Panel). The new-construction frame is used to provide coverage of new structures for which building permits have been issued since the last decennial census in areas covered by the unit frame. This frame is updated continually, and the percentage of addresses sampled from it increases each year until data from another decennial census become available.
Within each sample PSU, the addresses in the sampling frames are grouped into clusters. The clusters are then sampled, and the selected cluster of addresses is included for interviewing.4 In the unit frame, the 1996 Panel had clusters of one housing unit; for prior panels, clusters of two neighboring addresses were used. In the area and group quarter frames, clusters are constructed with the expectation of four housing units or housing unit equivalents. With the area frame, the sampled clusters are visited by SIPP interviewers prior to the scheduled interviewing. The interviewers list all residential addresses within the selected clusters. With the new-construction frame, the 1996 Panel has a 50-50 mixture of four- and eight-unit clusters. Previously, clusters of four housing units were formed. No clustering is used with the coverage improvement frame.
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