The census provides data for more types of geographic areas than any other major data source. Most SMSAs consist of a city of 50,000 or more inhabitants and the county in which it is located. Contiguous counties are included in an SMSA if they are socially and economically integrated with the central county. The inclusion of contiguous counties in an SMSA is based primarily by census data on commuting patterns. These concentrations of population, such as cities, have legally prescribed boundaries, powers and functions.
Tracts generally have stable boundaries.
When census tracts are established, they are designed to be relatively homogenous areas with respect to population characteristics, economic status, and living conditions. Tract boundaries are established with the intention of being maintained over a long time so that statistical comparisons can be made from census to census. When sufficient population increases occur, tracts are subdivided. It is easy to recombine the new tracts for comparison with older tracts. Other changes have included combining two or more tracts and minor realignments of tract boundaries to reflect new or better recognized physical features.