The document discusses income elasticity of demand. It defines income elasticity of demand as the rate of change in quantity demanded of a good due to changes in consumer income. Income elasticity measures the responsiveness of demand to changes in income. There are three types of income elasticity: positive, negative, and zero. Positive income elasticity occurs when demand increases with increases in income. Negative income elasticity is when demand decreases with increases in income. Zero income elasticity means demand does not change with changes in income. The document also lists factors that can affect price elasticity of demand.