A sociologist

Orshansky developed the poverty threshold in 1964
She used 1950s household expenditure surveys which showed that a typical American family spent about one third of their income on food.

She then had the Department of Agriculture calculate a "food basket" which is defined as the minimal nutritional requirement for a healthy diet, and see how much money that would cost.

She took this amount and multiplied it by three to get the poverty threshold. Families below this income are considered to be poor.

This method is still used today.