In IB Economics we define elasticity as a measure of how responsive one variable is to changes in price or any of the variable's determinants. This section of the course examines four types of elasticity. Price elasticity of demand examines how responsive demand is to changes in the price of a good, cross price elasticity of demand examines the responsiveness of demand for a good when the price of a related good changes (complements or substitutes). Income elasticity of demand measures the responsiveness of demand for a good to changes in income. Price elasticity of supply examines the responsiveness of supply to changes in the price of a good.