% change in price of product
The above formula will usually give a negative value, due to the inverse nature of the relationship between price and quantity demanded. This is given, and explained, by the "law of demand”. For example, if the price of a product increases by 5% and quantity demanded decreases by 5%, therefore: PED = −5%
PED = −1
The PED is negative for the vast majority of goods and services, however, economists often refer to price elasticity of demand as a positive value (i.e., in absolute value terms). Here is a graph, showing all the possible outcomes of PED according to elastic, inelastic, perfectly elastic, perfectly inelastic and unitary.
As the difference between the two prices or quantities increases, the accuracy of the PED given by the formula decreases for a combination of two reasons. First, the PED for a product is not necessarily always constant. PED can vary at different points along the demand curve, due to its percentage nature. Elasticity is not the same thing as the slope of the demand curve, which is dependent on the units used for both price and quantity. Second, percentage changes are not symmetric, more like the percentage change between any two values depends on which one is chosen as the starting value and which as the ending value. For example, if quantity demanded...