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Census tracts, blocks and CDPs explained: why geography matters to data users
Summary
Forum presenters explained the differences between statistical and legal geographies, why census tracts are kept relatively stable (optimum population ~4,000), and how Boundary and Annexation Survey and PSAP keep local boundaries current.
Speakers distinguished statistical geographies (created by the Census Bureau for tabulation and analysis) from legal and administrative geographies (created by state or local legal action). The presenter explained that legal geographies are defined by ordinances or laws, while statistical geographies — such as census tracts, block groups and census‑designated places (CDPs) — are created to support consistent data tabulation and comparison.
The presenter said census tracts have an optimum population of about 4,000 people to allow comparable analysis across diverse areas, and described how block groups and blocks are used for collection purposes. The session also reviewed programs used to update geographies — the annual Boundary and Annexation Survey (BAS), the decennial Participant Statistical Areas Program (PSAP) and the redistricting data program — and used examples such as Hempstead (Nassau County) and data.census.gov to show how place names are used to retrieve local statistics.
