The United States Census Bureau reports data by various geographic units. The top-to-bottom relationship shown here represents containment: the nation contains states, states contain counties, counties contain tracts, tracts contain block groups, and block groups contain blocks. Many other non-nesting reporting units are not shown.
Census tracts are relatively small subdivisions of a county. They typically have between 1,000 and 8,000 inhabitants and vary in size. They are designed to be fairly homogeneous with respect to demographic and economic conditions.
A block group is a cluster of blocks within a tract. A block group typically has between 600 and 3,000 inhabitants.
A census block (commonly an ordinary city block) is an area bounded by visible features, such as streets or railroad tracks, or by invisible boundaries, such as city limits. A block centroid is a census block represented as a point rather than a polygon. A centroid is located in the geographic center of the block it represents and has the attributes of that block.
The Census Bureau conducts a new census every ten years. The latest one was conducted in 2010. Professional demographers estimate values for the intervening years.