## Price Elasticity of Demand and Price Elasticity of Supply

Both the demand and supply curve show the relationship between price and the number of units demanded or supplied. **Price elasticity** is the ratio between the percentage change in the quantity demanded (Qd) or supplied (Qs) and the corresponding percent change in price. The **price elasticity of demand** is the percentage change in the quantity *demanded* of a good or service divided by the percentage change in the price. The **price elasticity of supply** is the percentage change in quantity *supplied* divided by the percentage change in price.

Elasticities can be usefully divided into three broad categories: elastic, inelastic, and unitary. An **elastic demand** or **elastic supply** is one in which the elasticity is greater than one, indicating a high responsiveness to changes in price. Elasticities that are less than one indicate low responsiveness to price changes and correspond to **inelastic demand** or **inelastic supply**. **Unitary elasticities** indicate proportional responsiveness of either demand or supply, as summarized in this table.

### Elastic, Inelastic, and Unitary: Three Cases of Elasticity

If . . . | Then . . . | And It Is Called . . . |
---|---|---|

\(\text{% change in quantity}>\text{% change in price}\) | \(\cfrac{\text{% change in quantity}}{\text{% change in price}}>1\) | Elastic |

\(\text{% change in quantity}=\text{% change in price}\) | \(\cfrac{\text{% change in quantity}}{\text{% change in price}}=1\) | Unitary |

\(\text{% change in quantity}<\text{% change in price}\) | \(\cfrac{\text{% change in quantity}}{\text{% change in price}}<1\) | Inelastic |

**Note:** Before we get into the nitty gritty of elasticity, enjoy this article on elasticity and ticket prices at the Super Bowl.

To calculate elasticity, instead of using simple percentage changes in quantity and price, economists use the average percent change in both quantity and price. This is called the Midpoint Method for Elasticity, and is represented in the following equations:

\(\begin{array}{rcl}\text{% change in quantity}& =& \cfrac{{\mathrm{Q}}_{2}–{\mathrm{Q}}_{1}}{({\mathrm{Q}}_{2}+{\mathrm{Q}}_{1})/2}× 100\\ \text{% change in price}& =& \cfrac{{\mathrm{P}}_{2}–{\mathrm{P}}_{1}}{({\mathrm{P}}_{2}+{\mathrm{P}}_{1})/2}× 100\end{array}\)

The advantage of the is **Midpoint Method** is that one obtains the same elasticity between two price points whether there is a price increase or decrease. This is because the formula uses the same base for both cases.