Elasticity

Elasticity is a central concept in the theory of supply and demand. In this context, elasticity refers to how strongly the quantities supplied and demanded respond to various factors, including price and other determinants. One way to define elasticity is the percentage change in one variable (the quantity supplied or demanded) divided by the percentage change in the causative variable. For discrete changes this is known as arc elasticity, which calculates the elasticity over a range of values. In contrast, point elasticity uses differential calculus to determine the elasticity at a specific point. Elasticity is a measure of relative changes.

Often, it is useful to know how strongly the quantity demanded or supplied will change when the price changes. This is known as the price elasticity of demand or the price elasticity of supply, respectively. If a monopolist decides to increase the price of its product, how will this affect the amount of their good that customers purchase? This knowledge helps the firm determine whether the increased unit price will offset the decrease in sales volume. Likewise, if a government imposes a tax on a good, thereby increasing the effective price, knowledge of the price elasticity will help us to predict the size of the resulting effect on the quantity demanded.

Elasticity is calculated as the percentage change in quantity divided by the associated percentage change in price. For example, if the price moves from $1.00 to $1.05, and as a result the quantity supplied goes from 100 pens to 102 pens, the quantity of pens increased by 2%, and the price increased by 5%, so the price elasticity of supply is 2%/5% or 0.4.

Since the changes are in percentages, changing the unit of measurement or the currency will not affect the elasticity. If the quantity demanded or supplied changes by a greater percentage than the price did, then demand or supply is said to be elastic. If the quantity changes by a lesser percentage than the price did, demand or supply is said to be inelastic. If supply is perfectly inelastic;that is, has zero elasticity, then there is a vertical supply curve.

Short-run supply curves are not as elastic as long-run supply curves, because in the long run firms can respond to market conditions by varying their holdings of physical capital, and because in the long run new firms can enter or old firms can exit the market.

Elasticity in relation to variables other than price can also be considered. One of the most common to consider is income. How strongly would the demand for a good change if income increased or decreased? The relative percentage change is known as the income elasticity of demand.

Another elasticity sometimes considered is the cross elasticity of demand, which measures the responsiveness of the quantity demanded of a good to a change in the price of another good. This is often considered when looking at the relative changes in demand when studying complements and substitute goods. Complements are goods that are typically utilized together, where if one is consumed, usually the other is also. Substitute goods are those where one can be substituted for the other, and if the price of one good rises, one may purchase less of it and instead purchase its substitute.

Cross elasticity of demand is measured as the percentage change in demand for the first good divided by the causative percentage change in the price of the other good. For an example with a complement good, if, in response to a 10% increase in the price of fuel, the quantity of new cars demanded decreased by 20%, the cross elasticity of demand would be -2.0.

In a frictionless economy, the price and quantity in any market would be able to move to a new equilibrium position instantly, without spending any time away from equilibrium. Any change in market conditions would cause a jump from one equilibrium position to another at once. In real economic systems, markets don't always behave in this way, and markets take some time before they reach a new equilibrium position. This is due to asymmetric, or at least imperfect, information, where no one economic agent could ever be expected to know every relevant condition in every market. Ultimately both producers and consumers must rely on trial and error as well as prediction and calculation to find the true equilibrium of a market.