Since 1905 the Bureau of the Census has devised and applied concepts and criteria For delineating metropolitan areas and has published statistics describing them. The Office of Management and Budget (OMB) and its predecessor, the Bureau of the Budget. have defined metropolitan areas for use by federal agencies since introducing standard metropolitan areas (SMAs) for the 1950 census. In the late 20th century, new national United States settlement forms emerged, and distinctions between urban and rural areas or between metropolitan and nonmetropolitan areas lost both their theoretical and practical significance. A proposed system of national settlement areas, applied experimentally to eight diverse states, uses counties (towns and cities in New England) as basic statistical units, with each categorized by its population density as ranked within both its state and the nation. Relative population density alone is an adequate and appropriate surrogate for activity patterns and interaction among geographic units, approximating the patterns delineated by current practice, while blanketing the nation with nonoverlapping statistical aggregates.
