The Neighborhood Change Database is based on the geographic unit of the Census Tract. The Census Tract is the Census Bureau's statistical equivalent of a large neighborhood (with an average of about 4,000 people).

You can select any of the 4 geographical levels:

• Nation (United States)
• States (50 + District of Columbia)
• Counties (3,141)
• Tracts (depends upon the census 35-65,000)
• MSA/CMSA and PMSA (are also included for 2000)

The data will be presented at the tract level only, but each record has additional geographic identifiers for larger geographies (such as county, metropolitan areas, places) so it is possible for you to summarize the tracts in other database or statistical programs.

The 1970 and 1980 censuses did not have full tract coverage of the US. The tracts are predominantly located in urban areas. By 1990 the US was fully tracted and all parts of the country were covered.

The actual remapping procedure for converting data from 1970, 1980, and 1990 tracts to 2000 tract boundaries is quite complicated. Those wishing more a technical explanation of this task should consult Appendix J of the Data Users' Guide. The basic procedure was to use geographic information system (GIS) software to overlay the boundaries of 2000 tracts with those of an earlier year. This allowed us to identify how tract boundaries had changed between censuses. We then used 1990 block data to determine the proportion of persons in each earlier tract that went into making up the new 2000 tract. For example, if a 1990 tract split into two tracts for 2000, the population may not have been divided evenly. Our method allows us to determine the exact weight to allocate to each portion.

These population weights were then applied to the various 1970, 1980, and 1990 tract level NCDB variables to convert them to 2000 tract boundaries. The population weights were used to convert all variables based on counts of persons, households, and housing units, all counts based on subpopulations (such as black persons or elderly households), and all aggregate data (such as aggregate household income). Proportions (such as the proportion of Hispanic persons) were remapped by first converting the respective numerator and denominator values (Hispanic persons and total persons, respectively) and then recalculating the proportion.