Table 2. The coding system that was used in Models 2 and 3.

	USDA Economic Research Service Rural-Urban Continuum Codes (RUCC) a		HCUPNET ADHD (patient residence)
Model Set	Code	Description
k = 1	j = 0	Central counties in metro areas of 1 million population or more (1993 only)	Large central metro
	j = 0
	j = 1	(Fringe, only 1993) counties in metro areas of 1 million population or more	Large central metro
	j = 1
k = 2	j = 2	Counties in metro areas of 250,000 to 1 million population	Large fringe metro
	j = 3	Counties in metro areas of fewer than 250,000 population	Large fringe metro
k = 3	j = 4	Urban population of 20,000 or more, adjacent to a metro area	Medium/Small Metro
	j = 4
	j = 5	Urban population of 20,000 or more, not adjacent to a metro area	Medium/Small Metro
	j = 5
	j = 6	Urban population of 2,500 to 19,999, adjacent to a metro area	Medium/Small Metro
	j = 6
	j = 7	Urban population of 2,500 to 19,999, not adjacent to a metro area	Medium/Small Metro
	j = 7
k = 4	j = 8	Completely rural or less than 2,500 urban population, adjacent to a metro area	Micropolitan and noncore (rural)
	j = 8
	j = 9	Completely rural or less than 2,500 urban population, not adjacent to a metro area	Micropolitan and noncore (rural)
	j = 9

^a Continuum codes changed from 1993 to 2003.