Interpreting Price Elasticity of Demand
Clarifying Price Elasticities
1. Perfectly inelastic
a. When quantity demanded does not respond at all to changes in price
i. Demand curve is a vertical line
ii. Price elasticity equals 0
2. Perfectly elastic
a. When any price increase will cause the quantity demanded to drop to zero.
i. Demand curve is horizontal
ii. This implies an infinite price elasticity of demand
Perfectly inelastic and perfectly elastic are two extremes. Most goods are somewhere in between.
3. Elastic
a. Price elasticity of demand is greater than 1
4. Inelastic
a. Price elasticity of demand is less than 1
5. Unit elastic
a. Price elasticity of demand is exactly 1
Why does it matter whether demand is elastic, inelastic or unit elastic?
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It helps predict how changes in price will affect total revenue, earned by producers from the sale
of that good.
o Total Revenue = P X Q
Also two countervailing effects are present
o A price effect
 After a price increase, each unit sold sells at a higher price, which tends to raise
revenue
o A quantity effect
 After a price increase, fewer units are sold, which tends to lower revenue
What factors determine Price Elasticity of Demand?
1. Whether close substitutes are available
a. Price elasticity of demand tends to be high if there are other goods that consumers
regard as similar and would be willing to consume instead
b. Price elasticity of demand tends to be low if there are no close substitutes
2. Whether a good is a necessity or a luxury?
a. Price elasticity tends to be low if it is something you must have
b. Price elasticity tends to be high if the good is a luxury
3. Share of income spent on the good
a. Price elasticity of demand tends to be low when spending on a good accounts for a
small share of a consumer’s income
i. A significant change in price has little impact on how much a consumer spends
b. When the good accounts for a significant share of consumer spending, consumer is less
likely to be responsive to a change in price thus price elasticity of demand is high
4. Time
a. Price elasticity of demand tends to increase as consumers have more time to adjust to a
price change
Long-run price elasticity of demand is higher than short-run elasticity.
Other Elasticities
Cross-Price Elasticity of Demand
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Demand for a good is often affected by the prices on other goods (substitutes and
complements)
A change in the price of related good shifts the demand curve of the original good, reflecting a
change in the quantity demanded at any given price.
This can be measured by the cross-price elasticity of demand
o It is a ratio of the percent change in the quantity demanded of one good to the percent
change in the price of another.
If two goods are substitutes (i.e. hot dogs/ hamburgers) cross-price elasticity of demand is positive
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If they are close substitutes cross-price elasticity will be positive and large
If not cross-price elasticity will be positive and small
When two goods are complements (i.e. hot dogs/ hot dog buns) cross-price elasticity is negative
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If the cross-price elasticity is slightly below zero, they are weak complements, if it is very
negative, they are strong complements
In the case of cross price elasticity the plus or minus sign is very important: it indicates whether the
goods are substitutes or complements
Income Elasticity of Demand
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Measures how changes in income affect the demand for a good
Income elasticity of demand can be both positive and negative
1. When the income elasticity of demand is positive, the good is a normal good- the quantity
demanded at any given price increases as income increases
2. When it is negative, the good is an inferior good- quantity demanded at any given price
decreases as income increases
Economist use income-elasticity of demand to predict which industries will grow most rapidly as
incomes of consumers grow over time.
This helps make a useful distinctions among normal goods
1. Income elastic
a. The income elasticity of demand for that good is greater than 1
i. When income rises, the demand for income-elastic goods rise faster than
income
1. Example: second homes and international travel
2. Income inelastic
a. The income elasticity of demand for that good is positive but less than 1
i. When income rises, demand for income-inelastic goods rises, but more slowly
than income
1. Example: food and clothing
Price Elasticity of Supply
Definition: A measure of the responsiveness of the quantity of a good supplied to the price of that good.
It is the ratio of the percent change in quantity supplied to the percent change in the price as we move
along the supply curve.
Formula
Extreme Values
1. Perfectly Inelastic Supply = 0
a. Graphed Vertically
2. Perfectly elastic Supply = infinite
a. Graphed horizontally
Factors that determine Price Elasticity of Supply
1. Availability of Inputs
a. Price elasticity of supply tends to be large when inputs are readily available and can be
shifted into and out of production at a relatively low cost
b. Price elasticity of supply tends to be small when inputs are available only in a more-orless fixed quantity or can be shifted into and out of production only at a relatively high
cost
2. Time
a. Price elasticity of supply tends to grow larger as producers have more time to respond
to a price change
i. Long-run price elasticity of supply is often higher than short run elasticity
Short run = a few weeks or months
Long run = several years