Elasticity of demand refers to the responsiveness of quantity demanded to changes in price. Price elasticity of demand is measured as the percentage change in quantity demanded divided by the percentage change in price. It can be measured over a segment of the demand curve or at a single point. Factors that determine price elasticity include availability of substitutes, proportion of income spent on the good, nature of the good, and possibilities of postponing consumption. Price elasticity is important because it indicates how changes in price will affect total revenue and consumer expenditure.