An initially low value for this quantity can cause a J-curve effect when a currency depreciates. Balance of trade improves with currency depreciation if the sum of this value for imports and exports is greater than one, according to the Marshall-Lerner condition. According to Ramsey's formula, an excise tax should be inversely proportional to this 
An initially low value for this quantity can cause a J-curve effect when a currency depreciates. Balance of trade improves with currency depreciation if the sum of this value for imports and exports is greater than one, according to the Marshall-Lerner condition. According to Ramsey's formula, an excise tax should be inversely proportional to this quantity in order to reduce deadweight loss. Its reciprocal is equal to the Lerner Index, which measures market power for monopolies. Marginal revenue is equal to "price times one plus the reciprocal of this quantity". This quantity is only positive for Veblen and Giffen goods, and when it equals zero, buyers will pay any amount for a good. For 10 points, name this quantity which measures change in demand with respect to change in price.