Factors and Types of Demand Elasticity

Exam Tip: The Midpoint Method When calculating elasticity between two points, using simple percentage changes can give different results depending on whether you are moving from Point A to B or B to A. To avoid this, always use the Midpoint Formula, which uses the average of the starting and final values as the base.

Elasticity and Total Revenue

For sellers, understanding elasticity is vital for managing Total Revenue (TR), which is calculated as Price $\times$ Quantity ($P \cdot Q$).

• Inelastic Demand: The "price effect" dominates. If you raise the price, the slight drop in quantity sold is outweighed by the higher price per unit, so Total Revenue rises.
• Elastic Demand: The "quantity effect" dominates. If you raise the price, the significant drop in quantity sold outweighs the higher price per unit, so Total Revenue falls.
• Unit Elastic Demand: The price and quantity effects exactly balance each other, leaving Total Revenue unchanged.

Factors Determining Price Elasticity

Why are some goods more sensitive to price than others? Four primary factors determine the price elasticity of demand:

1. Availability of Close Substitutes: This is perhaps the most important factor. If many substitutes exist, consumers can easily switch brands if one price rises, making demand elastic. If there are few substitutes, demand is inelastic.
2. Necessity vs. Luxury: Necessities (like medicine) tend to have inelastic demand because consumers must buy them regardless of price. Luxuries (like designer clothing) have elastic demand because consumers can easily forgo them if prices rise.
3. Share of Income: Goods that take up a large portion of a consumer's budget (expensive items) tend to have more elastic demand. Small, inexpensive items (like salt) often have inelastic demand because price changes feel negligible.
4. Length of Time: Demand becomes more elastic over time. In the short run, consumers may not be able to change their habits immediately, but over time, they can find substitutes or adjust their behavior.

Cross-Price Elasticity of Demand

Cross-price elasticity of demand measures how sensitive the quantity demanded of one good (Good A) is to a change in the price of another good (Good B):

$\text{Cross-price elasticity of demand} = \frac{\% \text{ change in quantity demanded of Good A}}{\% \text{ change in price of Good B}}$

This metric allows us to identify the relationship between two products:

• Substitutes: These have a positive cross-price elasticity. If the price of Brand X cookies rises, the demand for Brand Y cookies will increase.
• Complements: These have a negative cross-price elasticity. If the price of milk rises, the demand for Oreos (which are often eaten with milk) will decrease.

Income Elasticity of Demand

Income elasticity of demand measures how sensitive the quantity demanded of a good is to changes in consumer income:

$\text{Income elasticity of demand} = \frac{\% \text{ change in quantity demanded}}{\% \text{ change in income}}$

We use this to distinguish between two types of goods:

1. Normal Goods: These have a positive income elasticity. As income rises, demand increases.
   • Income-elastic normal goods: Demand increases significantly as income rises (e.g., vacations).
   • Income-inelastic normal goods: Demand increases, but at a slower rate than income.
2. Inferior Goods: These have a negative income elasticity. As income rises, consumers switch to better alternatives, causing the demand for the inferior good to fall (e.g., fast food or ramen noodles).