The single largest cost for school districts is labor, and labor costs are likely to vary between districts independently of total enrollment. In particular, districts where the cost of living is higher can be expected to pay higher salaries. If they did not, they would probably have considerable difficulty attracting and keeping employees. To control for this variation in input costs, the model includes both the median asking price for homes and the median asking price for apartment rent. Data for this variable come from Census 2000, “Summary File 3.”[15]