Income elasticity of demand
Income elasticity of demand is the percentage change in quantity of a good demanded divided by the percentage change in average income. It tells how the demand for a good reacts to changes in the average income. Normal goods have a positive income elasticity, while inferior goods have a negative income elasticity.
Among goods with positive income elasticity, Necessities and more essential goods have the level of elasticity at less than 1, while luxury goods have very large elasticity.