Federal shortage areas use a ratio of around 1:3500 with adjustments if higher infant mortality rate, geographic or other barriers to care. The estimate of adequate numbers of FP docs vary from 1:2000 to 1:2500, the major factor is the age of the population served. One fp doc can only serve 800 elderly. State shortage areas are usually set even lower than this, especially in states with counties with fewer providers. This provides some reserve when communities lose physicians that are difficult to replace. It is also a function of small numbers. Losing one of your 4 physicians in a county of 10,000 drops you from normal ratios to federal shortage area ratios. That is the reason that states are more reasonable with their shortage area designations. It is possible to get a shortage area designated, particularly if a practitioner has left. Many communities fail to update their eligibility and lose out on potential recruits, loan repayments, scholarships, grants, etc. It is also possible to get a specific part of a county designated, or to meet the specific needs of a population such as the Medicaid/uninsured population, specific geographic needs, correctional facilities, etc.
You can get the latest Federal Office of Shortage Designation info at: http://bphc.hrsa.gov/databases/newhpsa/newhpsa.cfm
COGME August 2000 for workforce numbers
Definitions of Rural
Bureau of Census - rural areas are those not categorized as urban. Urban areas are a city or cities of over 50,000 people in the closely settled contiguous area or those in places of over 2500 outside of urban areas
Office of Management and Budget - Metro is counties with towns of 50,000 or counties with an urbanized area of counties with a pop over 100,000 Nonmetro - those not in metro areas
Other definitions: See various federal sites and works. Some are based on economic and geographic situation rating 1-10 (Appalachian Regional Commission and others). Also see
A common rural Family Medicine definition - from the literature (mine and others) and categories of locations of graduates, towns of less that 25,000, not adjacent to a metro area are considered rural. AAFP categories fit with this definition. AAMC has a bit different categories.
See Ricketts for more info on Definitions and Shortage Areas.