SSI's experienced staff of sampling consultants will work with researchers to determine the most appropriate sampling design based on the objectives of the research project. Samples may be coded with up to 14 characters of geographic codes. SSI samples are generated using stratified sampling procedures, which divide the population of sampling units into subpopulations called strata. A separate sample is then selected from the units in each stratum. Stratification provides tighter sampling process controls in terms of selection and rounding errors and offers more flexibility in terms of allocation options.
Stratification also reduces sampling variance as compared to a simple random sample.
SSI's database has been stratified by county. Every exchange has been assigned to the county of plurality of listed residences. Stratification by county is used for several reasons. County-level stratification allows users easily to alter sampling rates across strata or within strata to meet specific study requirements or to control sample distribution geographically. The sample is then allocated proportionally across all strata using one of several frame adjustment options.