Key to this system are crosswalks for each prior year, serving the same purpose as the proprietary Block Weighting File developed by NCDB for 1990-2000. The crosswalk identifies what portion of a tract in one year should be allocated to a 2010 tract.
For every decennial year from 1970 to 2000, every row in the crosswalk lists a 2010 tract ID, the ID of a tract in the source year that contributes to it, and the share of the source tract's population attributes that should be allocated to the 2010 tract. In cases where there is an exact correspondence between the source tract and the 2010 tract, there is only one row of data for the 2010 tract. Otherwise there are as many rows as there are contributing tracts. (For completeness, the crosswalk file includes every contributing tract, regardless of how small a fraction of its population should be allocated to the 2010 tract. Therefore many 2010 tracts that we categorize as "no change" also list 2000 tracts that contribute to it, typically with very small weights.)
The crosswalk files include the tract ID in 2010 (which incorporates a state, county, and tract FIPS code), the 2010 metropolitan area (formally the Core Based Statistical Area or CBSA) code, and a flag to identify location in a central city in 2010 (referred to by the Census Bureau as a "principal city" in an MSA or metropolitan division. For the 2000-2010 crosswalk we provide one additional indicator that we believe will assist users of the interpolated data: whether there was a boundary change involving this tract and if so what type of change occurred between 2000 and 2010 (1 = no change; 2 = consolidation; 3 = split; 4 = many-to-many).
The method for creating the crosswalk is crucial to the accuracy of the estimates. It is described in more detail here.
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