For example, internal tests have concluded that expanding the service area of rural libraries to even a 10-mile radius does not significantly alter the poverty rate calculation for each library, nor does it tend to affect population enough in these areas to change their grant award levels. These variables include age, education level, family type, income, and race. Definitions Census blocks are small areas bounded on all sides by visible features such as streets, roads, streams, and railroad tracks, and by invisible boundaries such as city, town, township, and county limits, property lines, and short, imaginary extensions of streets and roads. A geographic block group is a cluster of blocks within a census tract or block numbering area. Block groups generally contain between 250 and 550 housing units, with the ideal size being 400 housing units.
Census tracts are small, relatively permanent statistical subdivisions of a county.
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. Self-Certification of Eligibility for Discounts, Library Eligibility, Determination of the Level of Low Income Discounts for Libraries. March 17, 1997.