GEOGRAPHIC SPINES IN THE 2020 CENSUS DISCLOSURE AVOIDANCE SYSTEM

The 2020 Census Disclosure Avoidance System (DAS) is a formally private mechanism that first adds independent noise to cross tabulations for a set of pre-specified hierarchical geographic units, which is known as the geographic spine. After post-processing these noisy measurements, the DAS outputs a formally private database with fields indicat-ing location in the standard census geographic spine, which is defined by the United States as a whole, states, counties, census tracts, block groups, and census blocks. This paper describes how the geographic spine used internally within the DAS to define the initial noisy measurements impacts accuracy of the output of the DAS. Specifically, tabulations for geographic areas tend to be most accurate for geographic areas that both 1) can be derived by aggregating together the geographic units of the internal spine other than block geographic units, and 2) are closer to the geographic units of the internal spine. After describing the methods supported by the DAS for defining the internal DAS geographic spine, we provide the settings used to define the 2020 Census production DAS executions. To demonstrate the accuracy impact of the choice of internal spine, we also provide accuracy metrics for three DAS executions using settings similar to the ones used for the 2020 Census Redistricting Data (P.L. 94-171) persons production DAS execution, but using three different choices of internal geographic spine.