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Price elasticity of demand (PED or Ed) is a measure used in economics to show the responsiveness, or elasticity, of the quantity demanded of a good or service to a change in its price. More precisely, it gives the percentage change in quantity demanded in response to a one percent change in price (ceteris paribus, i.e. holding constant all the other determinants of demand, such as income). It was devised by Alfred Marshall.
Price elasticities are almost always negative, although analysts tend to ignore the sign even though this can lead to ambiguity. Only goods which do not conform to the law of demand, such as Veblen and Giffen goods, have a positive PED. In general, the demand for a good is said to be inelastic (or relatively inelastic) when the PED is less than one (in absolute value): that is, changes in price have a relatively small effect on the quantity of the good demanded. The demand for a good is said to be elastic (or relatively elastic) when its PED is greater than one (in absolute value): that is, changes in price have a relatively large effect on the quantity of a good demanded.
Revenue is maximized when price is set so that the PED is exactly one. The PED of a good can also be used to predict the incidence (or "burden") of a tax on that good. Various research methods are used to determine price elasticity, including test markets, analysis of historical sales data and conjoint analysis.
The above formula usually yields a negative value, due to the inverse nature of the relationship between price and quantity demanded, as described by the "law of demand". For example, if the price increases by 5% and quantity demanded decreases by 5%, then the elasticity at the initial price and quantity = −5%/5% = −1. The only classes of goods which have a PED of greater than 0 are Veblen and Giffen goods. Because 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).
This measure of elasticity is sometimes referred to as the own-price elasticity of demand for a good, i.e., the elasticity of demand with respect to the good's own price, in order to distinguish it from the elasticity of demand for that good with respect to the change in the price of some other good, i.e., a complementary or substitute good. The latter type of elasticity measure is called a cross-price elasticity of demand.
As the difference between the two prices or quantities increases, the accuracy of the PED given by the formula above decreases for a combination of two reasons. First, the PED for a good is not necessarily constant; as explained below, 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; instead, 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 increases from 10 units to 15 units, the percentage change is 50%, i.e., (15 − 10) ÷ 10 (converted to a percentage). But if quantity demanded decreases from 15 units to 10 units, the percentage change is −33.3%, i.e., (10 − 15) ÷ 15.
Two alternative elasticity measures avoid or minimise these shortcomings of the basic elasticity formula: point-price elasticity and arc elasticity.
One way to avoid the accuracy problem described above is to minimise the difference between the starting and ending prices and quantities. This is the approach taken in the definition of point-price elasticity, which uses differential calculus to calculate the elasticity for an infinitesimal change in price and quantity at any given point on the demand curve: 
In other words, it is equal to the absolute value of the first derivative of quantity with respect to price (dQd/dP) multiplied by the point's price (P) divided by its quantity (Qd).
In terms of partial-differential calculus, point-price elasticity of demand can be defined as follows: let be the demand of goods as a function of parameters price and wealth, and let be the demand for good . The elasticity of demand for good with respect to price is
However, the point-price elasticity can be computed only if the formula for the demand function, , is known so its derivative with respect to price, , can be determined.
A second solution to the asymmetry problem of having a PED dependent on which of the two given points on a demand curve is chosen as the "original" point and which as the "new" one is to compute the percentage change in P and Q relative to the average of the two prices and the average of the two quantities, rather than just the change relative to one point or the other. Loosely speaking, this gives an "average" elasticity for the section of the actual demand curve—i.e., the arc of the curve—between the two points. As a result, this measure is known as the arc elasticity, in this case with respect to the price of the good. The arc elasticity is defined mathematically as:
This method for computing the price elasticity is also known as the "midpoints formula", because the average price and average quantity are the coordinates of the midpoint of the straight line between the two given points. However, because this formula implicitly assumes the section of the demand curve between those points is linear, the greater the curvature of the actual demand curve is over that range, the worse this approximation of its elasticity will be.
Together with the concept of an economic "elasticity" coefficient, Alfred Marshall is credited with defining PED ("elasticity of demand") in his book Principles of Economics, published in 1890. He described it thus: "And we may say generally:— the elasticity (or responsiveness) of demand in a market is great or small according as the amount demanded increases much or little for a given fall in price, and diminishes much or little for a given rise in price". He reasons this since "the only universal law as to a person's desire for a commodity is that it diminishes... but this diminution may be slow or rapid. If it is slow... a small fall in price will cause a comparatively large increase in his purchases. But if it is rapid, a small fall in price will cause only a very small increase in his purchases. In the former case... the elasticity of his wants, we may say, is great. In the latter case... the elasticity of his demand is small." Mathematically, the Marshallian PED was based on a point-price definition, using differential calculus to calculate elasticities.
The overriding factor in determining PED is the willingness and ability of consumers after a price change to postpone immediate consumption decisions concerning the good and to search for substitutes ("wait and look"). A number of factors can thus affect the elasticity of demand for a good:
Elasticities of demand are interpreted as follows:
|Perfectly inelastic demand|
|Inelastic or relatively inelastic demand|
|Unit elastic, unit elasticity, unitary elasticity, or unitarily elastic demand|
|Elastic or relatively elastic demand|
|Perfectly elastic demand|
A decrease in the price of a good normally results in an increase in the quantity demanded by consumers because of the law of demand, and conversely, quantity demanded decreases when price rises. As summarized in the table above, the PED for a good or service is referred to by different descriptive terms depending on whether the elasticity coefficient is greater than, equal to, or less than −1. That is, the demand for a good is called:
As the two accompanying diagrams show, perfectly elastic demand is represented graphically as a horizontal line, and perfectly inelastic demand as a vertical line. These are the only cases in which the PED and the slope of the demand curve (∆P/∆Q) are both constant, as well as the only cases in which the PED is determined solely by the slope of the demand curve (or more precisely, by the inverse of that slope).
A firm considering a price change must know what effect the change in price will have on total revenue. Revenue is simply the product of unit price times quantity:
Generally any change in price will have two effects:
For inelastic goods, because of the inverse nature of the relationship between price and quantity demanded (i.e., the law of demand), the two effects affect total revenue in opposite directions. But in determining whether to increase or decrease prices, a firm needs to know what the net effect will be. Elasticity provides the answer: The percentage change in total revenue is approximately equal to the percentage change in quantity demanded plus the percentage change in price. (One change will be positive, the other negative.) The percentage change in quantity is related to the percentage change in price by elasticity: hence the percentage change in revenue can be calculated by knowing the elasticity and the percentage change in price alone.
Hence, as the accompanying diagram shows, total revenue is maximized at the combination of price and quantity demanded where the elasticity of demand is unitary.
It is important to realize that price-elasticity of demand is not necessarily constant over all price ranges. The linear demand curve in the accompanying diagram illustrates that changes in price also change the elasticity: the price elasticity is different at every point on the curve.
PEDs, in combination with price elasticity of supply (PES), can be used to assess where the incidence (or "burden") of a per-unit tax is falling or to predict where it will fall if the tax is imposed. For example, when demand is perfectly inelastic, by definition consumers have no alternative to purchasing the good or service if the price increases, so the quantity demanded would remain constant. Hence, suppliers can increase the price by the full amount of the tax, and the consumer would end up paying the entirety. In the opposite case, when demand is perfectly elastic, by definition consumers have an infinite ability to switch to alternatives if the price increases, so they would stop buying the good or service in question completely—quantity demanded would fall to zero. As a result, firms cannot pass on any part of the tax by raising prices, so they would be forced to pay all of it themselves.
In practice, demand is likely to be only relatively elastic or relatively inelastic, that is, somewhere between the extreme cases of perfect elasticity or inelasticity. More generally, then, the higher the elasticity of demand compared to PES, the heavier the burden on producers; conversely, the more inelastic the demand compared to PES, the heavier the burden on consumers. The general principle is that the party (i.e., consumers or producers) that has fewer opportunities to avoid the tax by switching to alternatives will bear the greater proportion of the tax burden. In the end the whole tax burden is carried by individual households since they are the ultimate owners of the means of production that the firm utilises (see Circular flow of income).
Among the most common applications of price elasticity is to determine prices that maximize revenue or profit.
If one point elasticity is used to model demand changes over a finite range of prices, elasticity is implicitly assumed constant with respect to price over the finite price range. The equation defining price elasticity for one product can be rewritten (omitting secondary variables) as a linear equation.
Similarly, the equations for cross elasticity for n products can be written as a set of n simultaneous linear equations.
This form of the equations shows that point elasticities assumed constant over a price range cannot determine what prices generate maximum values of ; similarly they cannot predict prices that generate maximum or maximum revenue.
Constant elasticities can predict optimal pricing only by computing point elasticities at several points, to determine the price at which point elasticity equals -1 (or, for multiple products, the set of prices at which the point elasticity matrix is the negative identity matrix).
If the definition of price elasticity is extended to yield a quadratic relationship between demand units () and price, then it is possible to compute prices that maximize , , and revenue. The fundamental equation for one product becomes
and the corresponding equation for several products becomes
In most situations, revenue-maximizing prices are not profit-maximizing prices. For example, if variable costs per unit are nonzero (which they almost always are), then a more complex computation of a similar kind yields prices that generate optimal profits.
In some situations, profit-maximizing prices are not an optimal strategy. For example, where scale economies are large (as they often are), capturing market share may be the key to long-term dominance of a market, so maximizing revenue or profit may not be the optimal strategy.
Various research methods are used to calculate price elasticities in real life, including analysis of historic sales data, both public and private, and use of present-day surveys of customers' preferences to build up test markets capable of modelling such changes. Alternatively, conjoint analysis (a ranking of users' preferences which can then be statistically analysed) may be used.
Though PEDs for most demand schedules vary depending on price, they can be modeled assuming constant elasticity. Using this method, the PEDs for various goods—intended to act as examples of the theory described above—are as follows. For suggestions on why these goods and services may have the PED shown, see the above section on determinants of price elasticity.