UAs for Puerto Rico use the zona urbana as a central place, and add to them the adjacent densely settled areas in the home municipio as well as those in adjacent municipios. Thus, although UAs must contain at least 50,000 people, there is no population requirement for zonas urbanas. Furthermore, the Planning Board delineated the zonas urbanas several years in advance of the 1990 census, based on compact settlement, while the Census Bureau delineated the UAs based on specific criteria applied to the time-of-the-census population counts and densities.
There are clusters of population in Puerto Rico that are distinct from the zonas urbanas. Population counts for a few villages appear in decennial census reports as early as the 1940 census. The same term was used in the 1950 census, and village-aldea appeared in the 1960 and 1970 censuses. For the 1980 census, the term for these CDP equivalents was shortened to only the Spanish aldea (literally, village); for the 1990 census, the Puerto Rico government redesignated them with the term comunidad (literally, community). An aldea/comunidad had to have a census population of at least 1,000 to be reported separately for the censuses, and its boundaries had to follow visible features and/or municipio or barrio boundaries.
Census tracts have been delineated in 56 of the 78 municipios; the other 22 municipios are divided into BNAs. The Census Bureau reported data by block for selected areas of Puerto Rico in the 1960, 1970, 1980, and 1990 censuses; for the 1990 census, data were available by block for the entire Commonwealth. For that census, the Planning Board delineated BGs that contained an optimum of 400 housing units and ranged from 250 to 550 housing units. These were divided during the 1990 census field operations to form enumerator assignments (address register areas), with 140 to 160 housing units. This paralleled the 1980 census requirement that an ED contain an optimum of 140 housing units or 550 people, a size that was designed to expedite the enumeration of the population.
In addition to the legal entities previously discussed, Puerto Rico is divided into election districts. To ensure the ability to tabulate data for these entities, the Census Bureau offered the opportunity for the Puerto Rico government
