The poorer a household, the bigger the share spent on food

Statistician Ernst Engel found that as household income rises, the percentage spent on food falls, even though the actual amount spent on food keeps increasing. This relationship, known as Engel's Law, still underpins how economists estimate poverty and compare living standards across countries. It was one of the first quantitative laws in consumer economics.

— Ernst Engel, Die Productions- und Consumtionsverhältnisse des Königreichs Sachsen — 1857 statistical study of Saxon household budgets