96PART TWOSUPPLY AND DEMAND I: HOW MARKETS WORKpercentage change in price. In this example, the percentage change in price is a pos-itive10 percent (reflecting an increase), and the percentage change in quantity de-manded is a negative20 percent (reflecting a decrease). For this reason, priceelasticities of demand are sometimes reported as negative numbers. In this bookwe follow the common practice of dropping the minus sign and reporting all priceelasticities as positive numbers. (Mathematicians call this the absolute value.) Withthis convention, a larger price elasticity implies a greater responsiveness of quan-tity demanded to price.THE MIDPOINT METHOD: A BETTER WAY TO CALCULATEPERCENTAGE CHANGES AND ELASTICITIESIf you try calculating the price elasticity of demand between two points on a de-mand curve, you will quickly notice an annoying problem: The elasticity frompoint A to point B seems different from the elasticity from point B to point A. Forexample, consider these numbers:Point A:
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