>CB: I'm trying to think how they draw the census tracts and whether
>if drawn differently , there would be a different result.
They're meant to be pretty homogenous. <http://www.census.gov/geo/www/cen_tract.html>:
>Census tracts are small, relatively permanent statistical
>subdivisions of a county. Census tracts are delineated for most
>metropolitan areas (MA's) and other densely populated counties by
>local census statistical areas committees following Census Bureau
>guidelines (more than 3,000 census tracts have been established in
>221 counties outside MA's). Six States (California, Connecticut,
>Delaware, Hawaii, New Jersey, and Rhode Island) and the District of
>Columbia are covered entirely by census tracts. Census tracts
>usually have between 2,500 and 8,000 persons and, when first
>delineated, are designed to be homogeneous with respect to
>population characteristics, economic status, and living conditions.
>Census tracts do not cross county boundaries. The spatial size of
>census tracts varies widely depending on the density of settlement.
>Census tract boundaries are delineated with the intention of being
>maintained over a long time so that statistical comparisons can be
>made from census to census. However, physical changes in street
>patterns caused by highway construction, new development, etc., may
>require occasional revisions; census tracts occasionally are split
>due to large population growth, or combined as a result of
>substantial population decline. Census tracts are referred to as
>"tracts" in all 1990 data products. Block numbering areas (BNA's)
>are small statistical subdivisions of a county for grouping and
>numbering blocks in non-metropolitan counties where local census
>statistical area committees have not established census tracts. More
>information is available in the Geographic Areas Reference Manual.