Consumer's Surplus as an Exact Approximation When Prices Are Appropriately Deflated

A canonical price-normalized form is proposed as a generalization of the ordinary consumer's surplus expression commonly used to evaluate changes in economic welfare. This familiar-looking formula, it is proved, can be rigorously interpreted as representing the first- and second-order terms of a Taylor-series expansion for the equivalent-variation or willingness-to-pay function of a single consumer. In principle, the lowly consumer's surplus triangle-and-rectangle methodology can be rigorously defended as an exact approximation to a theoretically meaningful measure as long as prices are appropriately deflated. The appropriate price deflator is derived, and some implications are discussed.