Census Block Groups
Contents
A census block group is a geographic unit used by the U.S. Census Bureau that is an intermediate level of geography between the census tract and the census block. They are essentially clusters of blocks within the same census tract.
Here's a breakdown of their key characteristics:
| • | Population Size: A census block group is designed to have a population of 600 to 3,000 people. This is a larger unit than an individual census block, but a smaller one than a census tract. |
| • | Hierarchical Structure: Block groups are a core component of the census geographic hierarchy. Every census tract is subdivided into one or more block groups, and every block group, in turn, is made up of a cluster of census blocks. Block groups never cross the boundaries of a census tract or a county. |
| • | Data Availability: The most significant function of block groups is that they are the smallest geographic unit for which the Census Bureau publishes sample data. This refers to data collected from a fraction of all households, primarily through the American Community Survey (ACS). This includes detailed information on topics like income, education, housing characteristics, and employment. While "100-percent data" (basic demographics) is available at the even smaller census block level, the sample data requires a larger population to ensure statistical reliability and protect the privacy of individuals. |
| • | Identification: Each block group has a unique code. The first digit of the block number within a census tract typically corresponds to the block group number. For example, all blocks numbered from 3001 to 3999 in a given census tract would belong to block group 3. |
| • | Usefulness: Because they are the smallest unit for which a wealth of detailed data is available, block groups are widely used in research, marketing, urban planning, and policy analysis to understand characteristics of small neighborhoods. |