The American Community Survey one-year releases only include geographies with 65,000 or more population.u00a0 The three-year releases only include geographies with 20,000 or more population. Only the 5-year releases attempt to provide estimates for Census geographies of any population (down to the block group level, but not the block).So it is the case that only about 25% of US counties are included in the 1-year releases, and about 60% are included in the 3-year releases. This page gives some more information: When to use 1-year, 3-year, or 5-year estimatesu00a0 Consider also reading one of the Handbooks for Data Usersu2014they're basically the same, but some are fine-tuned to specific kinds of users, and, as far as these things go, they are pretty readable.