The current poverty measure was established in the 1960s and is now widely acknowledged to be flawed.2 It was based on research indicating that families spent about one-third of their incomes on food – the official poverty level was set by multiplying food costs by three. Since then, the figures have been updated annually for inflation but have otherwise remained unchanged.
[…]
Food now comprises only one-seventh of an average family’s expenses, while the costs of housing, child care, health care, and transportation have grown disproportionately. Thus, the poverty level does not reflect the true cost of supporting a family. In addition, the current poverty measure is a national standard that does not adjust for the substantial variation in the cost of living from state to state and between urban and rural areas.

National Center for Children in Poverty: Measuring Poverty in the United States (May 2009)

Personal note: and there’s little incentive now, to change the system of calculations.

(via aegipan-omnicorn)