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 The Census Bureau also requests that at the time each census tract/BNA is established, it contain (if possible) a population whose housing and socioeconomic characteristics are similar. Because the characteristics of neighborhoods and other small areas change with time, census tracts/BNAs may become less homogeneous in succeeding censuses.

To facilitate data processing and publication, the Census Bureau identifies census tracts and BNAs by number rather than name. Each census tract has a basic census tract number composed of no more than four digits, and may have an optional two-digit decimal suffix. (Leading zeroes appear in electronic media products, but do not appear on the Census Bureau’s maps or in the printed reports.) All BNAs have a four-digit basic number and may have an optional two-digit decimal suffix. The Census Bureau uses the numbers 1 to 9499.99 to identify census tracts, and 9501 to 9989.99 to identify BNAs.

Sometimes the Census Bureau recommends a range or series of census tract/BNA numbers to the census statistical areas committee or agency participating in the BNA program to avoid duplication with adjoining counties. For example, if two counties in the same MA both contain census tracts numbered 101 through 110, the Census Bureau might recommend that one county renumber their census tracts 1101 through 1110, and the other renumber theirs 2101 through 2110.

A permanent numbering system is desirable since it helps data users make intercensal comparisons of information by census tract. Census tract updates often involve the subdivision of an existing census tract (or census tracts) into two or more new units. When new census tracts (splits) occur within an established set of census tracts, the Census Bureau recommends retaining the original four-digit census tract number and adding a two-digit decimal suffix. As a result, Census Tract 101 may be split into Census Tracts 101.01, 101.02, and so forth, depending upon how many new census tracts are created. If a census tract identified by a suffixed number is subsequently split, the census statistical areas committee usually drops the existing suffix and utilizes the next available suffixes. Census Tracts and Block Numbering Areas10-7