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Census geography primer: tracts, block groups, blocks and PUMAs
Summary
Census geographies are hierarchical: blocks → block groups → tracts → counties → states; presenters explained when each summary level is available and recommended PUMAs when users need microdata at ~100,000-person scale.
Presenters described the Census Bureau's geographic hierarchy and how it affects data availability. "We divide all of our counties into census tracts. Those census tracts are divided even further into block groups and those block groups are divided even further down to census blocks," Kira Obradovich said, summarizing the nested structure.
Obradovich recommended that users select the coarsest geography that meets their needs because more-detailed geographies often reduce the surveys and years available. She noted PUMAs (Public Use Microdata Areas) as a useful alternative when smaller place-level geographies are not supported in microdata: "These are areas of a 100,000 or more within each state," she said. The presentation also covered mapping in data.census.gov so users can visualize which block groups and tracts comprise a neighborhood and export or save selections for analysis.
The presenters also summarized practical limits: ACS tables and decennial data typically persist to tract level, while some high-frequency surveys (for example, the Household Pulse Survey) are limited to national, state or larger metro statistical area levels in data.census.gov. Users were advised to consult survey pages for definitive summary-level availability and to contact a data dissemination specialist for help when local geography is required.
