A measure of the responsiveness of one variable to changes in another. Economists have identified four main types. * price elasticity measures how much the quantity of supply of a good, or demand for it, changes if its price changes. If the percentage change in quantity is more than the percentage change in price, the good is price elastic; if it is less, the good is inelastic. * income elasticity of demand measures how the quantity demanded changes when income increases. * Cross-elasticity shows how the demand for one good (say, coffee) changes when the price of another good (say, tea) changes. If they are substitute goods (tea and coffee) the cross-elasticity will be positive: an increase in the price of tea will increase demand for coffee. If they are complementary goods (tea and teapots) the cross-elasticity will be negative. If they are unrelated (tea and oil) the cross-elasticity will be zero. * Elasticity of substitution describes how easily one input in the production process, such as labor, can be substituted for another, such as machinery.