[us16]  Are defined as rural areas which are not classified as Metropolitan Statistical Areas (i.e. a county or group of contiguous counties which contains least one city of 50,000 inhabitants or more, or "twin cities" with a combined population of at least 50,000).
[us16]  An MSA is a county or group of contiguous counties which contains least one city of 50,000 inhabitants or more, or "twin cities" with a combined population of at least 50,000. In addition to the county, or counties, containing such a city or cities, contiguous counties are included in an SMSA if, according to certain criteria, they are essentially metropolitan in character and are socially and economically integrated with the central city. The largest city in each MSA is a central city.