Blocks are numbered uniquely within each census tract and consist of three digits. If a block is split by a jurisdictional boundary, a single-letter suffix will be added. There are approximately 20,000 blocks in the San Diego region. A block group is a contiguous cluster of blocks that have the sam first digit of the three-digit identifying number within a census tract. There are approximately 1,600 block groups in the region.
Census tracts are small, relatively homogeneous statistical subdivisions of a county.
Census tracts are delineated for all metropolitan areas and other densely populated areas by local Statistical Areas Committees following Bureau of the Census guidelines. There were 437 census tracts in the region for the 1990 Census. Census 2000 has 605 census tracts. There are 41 SRAs in the region. Except for minor census tract boundary changes, SRA boundaries do not change.