Chapter 9 — Front Matter
Purpose and Overview
Markets communicate through prices. When the price of gasoline rises, some drivers cut back on long trips. When the price of a concert ticket doubles, some fans decide to stay home. When the price of insulin rises, most diabetic patients continue buying it because they have no choice.
The question economists ask is not simply whether buyers and sellers respond to price changes — they almost always do. The question is how much they respond. A small price increase might cause a huge drop in sales for one product and barely any drop for another. A tax on cigarettes might raise enormous government revenue or almost none, depending on how responsive smokers are to higher prices.
Elasticity is the economic concept that measures this responsiveness. It turns a qualitative question — "Do buyers respond to price?" — into a quantitative one: "By how much, exactly?" Businesses use it to set prices and predict revenue. Governments use it to estimate the effects of taxes and regulations. Farmers, pharmaceutical companies, retailers, labor unions, and tax authorities all need to understand elasticity to make good decisions.
Opening Question
Think of one product you would keep buying even if its price doubled, and one product you would stop buying after even a small price increase. What makes those two products different? By the end of this chapter, you will have precise language and a formula for explaining exactly why.
10Sections
4Elasticity Types
22Key Terms
29Practice Problems
Learning Objectives
- 1Explain what elasticity measures and why it is expressed as a ratio of percentage changes.
- 2Calculate price elasticity of demand from price and quantity data.
- 3Classify demand as elastic, inelastic, unit elastic, perfectly elastic, or perfectly inelastic.
- 4Identify the determinants of demand elasticity, including substitutes, necessity, budget share, and time horizon.
- 5Use elasticity to predict how a price change will affect a seller's total revenue.
- 6Calculate price elasticity of supply and identify its key determinants.
- 7Calculate income elasticity of demand and classify goods as normal, inferior, necessity, or luxury.
- 8Calculate cross-price elasticity of demand and classify goods as substitutes or complements.
- 9Apply elasticity reasoning to business pricing, tax incidence, agricultural income, and public policy debates.
- 10Identify common errors in elasticity reasoning and correct them using evidence and formulas.
Sections 9.1 – 9.4
Measuring How Buyers Respond to Price
What Elasticity Is · Price Elasticity of Demand · Categories · Determinants
Section 9.1
What Elasticity Measures
Suppose a local coffee shop raises the price of a medium coffee from $3.00 to $3.30 — a 10 percent increase. After the price change, the shop sells 8 percent fewer coffees per day. Was that response large or small? It depends on what you are comparing. Most regular coffee drinkers accepted the higher price without changing their habits.
Now suppose the same shop raises the price of bottled water from $1.00 to $1.50 — a 50 percent increase. After the price change, the shop sells 70 percent fewer bottles of water, because customers simply walk to the water fountain down the hall or buy water at the grocery store next door. The response to the water price increase was much larger, relative to the price change, than the response to the coffee price increase. Water customers are more price-responsive than coffee customers — at least in this setting.
Elasticity gives economists a way to measure and compare these responses precisely.
The General Formula
Elasticity = (Percentage change in the outcome variable) ÷ (Percentage change in the cause variable)
This ratio answers the question: for every 1 percent change in the cause, how many percent does the outcome change? For price elasticity of demand, the cause is price and the outcome is quantity demanded. For price elasticity of supply, the cause is price and the outcome is quantity supplied.
Why Percentages, Not Units
Expressing elasticity as a ratio of percentage changes makes it unit-free. Comparing "a change of 10 gallons" to "a change of 10 car stereos" is meaningless, because the goods are measured in different units and cost different amounts. Comparing "a 10 percent change in milk sales" to "a 10 percent change in car stereo sales" is perfectly sensible. Elasticity works across any product, market, currency, or time period because it always speaks in percentages.
Elasticity connects price changes to real-world outcomes. A firm raising its price needs to know whether it will earn more or less revenue afterward. A government imposing a tax needs to know whether buyers will absorb it or cut back sharply. A farmer facing falling crop prices needs to know whether producing more or less will improve income. In each case, the answer depends on elasticity.
- In your own words, explain why elasticity is calculated using percentage changes rather than raw unit changes.
- Describe one product you would keep buying even after a large price increase, and one you would stop buying after a small price increase. What does this difference illustrate?
- Write the general elasticity formula from memory, then explain what the "cause" and "outcome" variables are for price elasticity of demand.
Section 9.2
Price Elasticity of Demand
Price elasticity of demand measures how responsive the quantity demanded of a good is to a change in its price. It answers: when price changes by 1 percent, by what percentage does quantity demanded change?
Formula
E_d = (Percentage change in quantity demanded) ÷ (Percentage change in price)
Percentage change for any variable: (New value − Old value) ÷ Old value × 100
Example 9.2.1
Calculating price elasticity of demand for concert tickets
- SituationThe price of a concert ticket rises from $50 to $60, and quantity demanded falls from 1,000 tickets to 800 tickets.
- % Price($60 − $50) ÷ $50 × 100 = 20%
- % Quantity(800 − 1,000) ÷ 1,000 × 100 = −20%
- E_dE_d = −20% ÷ 20% = −1.0
Key takeaway: Price elasticity of demand for concert tickets at this point is 1.0 in absolute value — unit elastic.
Sign Convention
Price elasticity of demand is almost always negative, because demand curves slope downward. Economists usually report the absolute value to make comparisons easier. When this chapter says "elasticity equals 2.0," it means the absolute value is 2.0.
A Subtle Problem: Direction Matters
The basic percentage-change formula gives a different elasticity depending on whether you go from $50 to $60 or from $60 to $50, because the denominator changes. The midpoint method solves this by using the average of the two values as the base:
% Change in quantity = (Q₂ − Q₁) ÷ [(Q₁ + Q₂) ÷ 2] × 100
% Change in price = (P₂ − P₁) ÷ [(P₁ + P₂) ÷ 2] × 100
For introductory purposes, using the basic formula with a stated direction is acceptable. The midpoint method is presented here so you recognize it in more advanced settings.
- The price of a haircut rises from $20 to $24. Quantity demanded falls from 300 per week to 270 per week. Calculate price elasticity of demand.
- Explain why economists usually report the absolute value of price elasticity of demand rather than the negative number itself.
- Explain in your own words why the midpoint method gives the same elasticity regardless of whether price is rising or falling.
Section 9.3
Elastic, Inelastic, and Unit Elastic Demand
Not all demand responds to price in the same way. Economists classify demand into categories based on the elasticity value.
Elastic demand (E_d > 1): Quantity demanded changes by a larger percentage than price. Buyers are highly responsive — they have good substitutes, are not urgent about the purchase, or the item is not a large necessity. Examples: luxury clothing, vacation travel, restaurant meals, brand-name electronics.
Inelastic demand (E_d < 1): Quantity demanded changes by a smaller percentage than price. Buyers are not very responsive — they may have few substitutes, need the good urgently, or the purchase may be a small share of their budget. Examples: insulin, gasoline in the short run, cigarettes for addicted smokers, electricity, basic food staples.
Unit elastic demand (E_d = 1): Quantity demanded changes by exactly the same percentage as price. No real market is exactly unit elastic at all prices, but the concept is a useful dividing line between elastic and inelastic demand.
The Extreme Cases
Perfectly inelastic demand (E_d = 0): Quantity demanded does not change at all when price changes — a vertical demand curve. Rare, but life-saving medications with no alternatives come close.
Perfectly elastic demand (E_d = infinity): Buyers will buy any quantity at one price but none at a higher price — a horizontal demand curve. Individual wheat farmers face nearly perfectly elastic demand; they can sell any amount at the market price, but lose all buyers if they charge even a penny more.
| Elasticity Value (absolute) | Label | Meaning |
|---|
| 0 | Perfectly inelastic | Quantity does not respond at all |
| Between 0 and 1 | Inelastic | Quantity responds less than price |
| Exactly 1 | Unit elastic | Quantity responds equally to price |
| Greater than 1 | Elastic | Quantity responds more than price |
| Infinity | Perfectly elastic | Buyers buy all at one price, none above |
- A 10% price increase causes a 25% drop in quantity demanded. Classify.
- A 20% price increase causes a 4% drop in quantity demanded. Classify.
- A 15% price increase causes a 15% drop in quantity demanded. Classify.
- Sketch a vertical demand curve. Which category does it represent?
- Sketch a horizontal demand curve. Which category does it represent?
- Explain why true perfect inelasticity is rare in real markets.
Section 9.4
Determinants of Demand Elasticity
Why is demand elastic for some goods and inelastic for others? Several factors determine how responsive buyers are to price changes.
| Determinant | More Elastic When… | More Inelastic When… |
|---|
| Substitutes | Close substitutes are readily available | Few or no substitutes exist |
| Necessity vs. luxury | The good is a luxury | The good is a necessity |
| Share of budget | The good takes a large share of spending | The good is a tiny share of spending |
| Time horizon | Buyers have had a long time to adjust | Buyers must respond immediately, in the short run |
| Market definition | The market is narrowly defined (one brand) | The market is broadly defined (the whole category) |
Substitutes Are the Most Important Determinant
The demand for Coca-Cola is more elastic than the demand for cola drinks generally, which is more elastic than the demand for all beverages together. At the broadest category, fewer substitutes exist, so demand is less elastic. This is why a specific brand almost always has higher elasticity than its broader product category.
Example 9.4.1
Time horizon and gasoline demand
- Short runWhen gasoline prices rise sharply, drivers cannot immediately sell their cars and buy fuel-efficient models. They continue driving and paying — demand is inelastic.
- Long runOver years, drivers replace old vehicles with efficient ones, move closer to work, or switch to public transit. The long-run response is much larger — demand is more elastic.
Key takeaway: Demand tends to be more elastic in the long run than in the short run, because buyers have more time to adjust.
- Explain why demand for "food" is more inelastic than demand for "hamburgers," which is more inelastic than demand for "Quarter Pounders at one specific McDonald's location."
- A 50 percent increase in the price of salt and a 50 percent increase in rent affect household budgets very differently. Explain why, using the budget-share determinant.
- Choose one good you buy regularly. Identify which determinant most explains why your demand for it is relatively elastic or relatively inelastic.
Sections 9.6 – 9.7
Measuring How Sellers Respond to Price
Price Elasticity of Supply · Determinants of Supply Elasticity
Section 9.6
Price Elasticity of Supply
Price elasticity of supply measures how responsive the quantity supplied of a good is to a change in its price. Because supply curves slope upward — when price rises, quantity supplied rises — both percentage changes move in the same direction, so price elasticity of supply is positive.
Formula
E_s = (Percentage change in quantity supplied) ÷ (Percentage change in price)
| Category | Meaning |
|---|
| Elastic supply (E_s > 1) | Quantity supplied changes by more than price; sellers expand production easily |
| Inelastic supply (E_s < 1) | Quantity supplied changes by less than price; sellers cannot easily expand |
| Unit elastic supply (E_s = 1) | Quantity supplied changes by the same percentage as price |
| Perfectly inelastic supply (E_s = 0) | Quantity supplied does not change at all; vertical supply curve |
| Perfectly elastic supply (E_s = infinity) | Sellers supply any quantity at one price, none below it; horizontal supply curve |
Example 9.6.1
Calculating elasticity of supply for residential painting
- SituationThe price of residential painting rises from $2,000 to $2,500 per job — a 25% increase. The number of painters offering services rises from 80 to 96 — a 20% increase.
- E_sE_s = 20% ÷ 25% = 0.8
- InterpretationSupply is inelastic. Painters cannot expand their numbers quickly, even at higher prices — it takes time to find workers, train them, and acquire equipment.
Key takeaway: A value below 1 means sellers respond less than proportionally to the price change.
- The wage for construction workers rises from $25 to $30 per hour. The number of workers willing to work rises from 1,000 to 1,200. Calculate price elasticity of supply.
- Explain why price elasticity of supply is positive while price elasticity of demand is almost always negative.
- Sketch a perfectly inelastic supply curve. Give one real-world example where supply is close to this case in the very short run.
Section 9.7
Determinants of Supply Elasticity
Why is supply elastic for some goods and inelastic for others? Several factors determine how responsive sellers are.
| Determinant | More Elastic When… | More Inelastic When… |
|---|
| Time horizon | Sellers have had a long time to expand capacity | Sellers must respond immediately, in the short run |
| Availability of inputs | Inputs are readily available and quick to obtain | Inputs are scarce, specialized, or slow to produce |
| Flexibility of production | Resources can shift easily between uses | Production is highly specialized |
| Inventory and storage | Goods can be stored and released as needed | Goods are perishable and cannot be stored |
| Entry of new firms | New firms can enter easily | Entry requires large capital, rare resources, or approvals |
Time Is the Most Important Determinant
In the short run, sellers are constrained by fixed resources — factory size, equipment, land, and specialized workers. In the long run, they can build new factories, hire and train workers, acquire more land, and adopt new technology. As a result, supply is almost always more elastic in the long run than in the short run. When oil prices rise sharply, oil companies cannot immediately open new wells; over years, they invest in new exploration, drilling, and refining capacity.
Software Versus Wine
A software company can add digital copies of its product almost instantly at low cost — highly elastic supply. A vineyard producing premium wine cannot quickly expand because premium grapes take years to grow — very inelastic supply. The same logic of input availability explains both cases.
- Explain why agricultural commodities like wheat tend to have more inelastic supply in the short run than manufactured goods like furniture.
- Why does the ability to hold inventory increase the elasticity of supply for a good, while perishability decreases it?
- Choose an industry you are familiar with. Identify one determinant that makes its supply more elastic or more inelastic.
Sections 9.8 – 9.10
Income, Cross-Price, and Real-World Applications
Income Elasticity · Cross-Price Elasticity · Business and Public Policy
Section 9.8
Income Elasticity at an Introductory Level
Not all demand responses are about price. Income also matters. When household incomes rise, spending patterns change. Income elasticity of demand measures how responsive quantity demanded is to a change in consumer income.
Formula
Income Elasticity = (Percentage change in quantity demanded) ÷ (Percentage change in income)
| Income Elasticity | Type of Good | Example |
|---|
| Negative | Inferior good | Instant noodles, low-quality generic products |
| Between 0 and 1 | Normal good (necessity) | Food staples |
| Greater than 1 | Normal good (luxury) | Foreign travel, yachts, private schools |
When income rises 10 percent and quantity demanded of restaurant meals rises 15 percent, income elasticity is 1.5 — restaurant meals are a luxury. As incomes rise, consumers can afford better alternatives and switch away from inferior goods, so demand for those goods falls even as overall spending power increases.
- Household income rises by 15 percent. Demand for air travel rises by 30 percent. Calculate income elasticity. Is air travel a normal good, inferior good, necessity, or luxury?
- A country's incomes rise by 20 percent, and sales of low-quality generic canned food fall by 10 percent. Calculate income elasticity for this canned food and classify the good.
- Give one example of a good you believe is inferior in your own household's spending and explain your reasoning.
Section 9.9
Cross-Price Elasticity at an Introductory Level
Demand for one good can respond to price changes in other goods — particularly substitutes and complements. Cross-price elasticity of demand measures how responsive the quantity demanded of one good is to a change in the price of a different good.
Formula
Cross-Price Elasticity = (% Change in quantity demanded of Good A) ÷ (% Change in price of Good B)
| Cross-Price Elasticity | Relationship | Example |
|---|
| Positive | Substitutes | Butter and margarine |
| Near zero | Unrelated goods | Shoes and motor oil |
| Negative | Complements | Smartphones and smartphone cases |
Example 9.9.1
Substitutes versus complements
- SubstitutesIf the price of butter rises 20% and quantity demanded of margarine rises 10%, cross-price elasticity is +0.5. Butter and margarine are substitutes.
- ComplementsIf the price of smartphones rises 15% and quantity demanded of smartphone cases falls 12%, cross-price elasticity is −0.8. Smartphones and cases are complements.
Key takeaway: The sign tells you the relationship; the size tells you how strong that relationship is.
- The price of printer ink rises by 10 percent. Demand for printers falls by 8 percent. Calculate cross-price elasticity. Are printers and ink substitutes or complements?
- Name a pair of goods you believe are substitutes and a pair you believe are complements. Predict the sign of cross-price elasticity for each pair.
- Explain why cross-price elasticity can apply to goods that are not obviously related, using an example from the chapter.
Section 9.10
Elasticity in Business and Public Policy
Elasticity is not just theoretical — it has direct, practical applications in pricing decisions, tax policy, agriculture, labor markets, and drug policy.
Business Pricing
Firms want to know their demand elasticity before changing prices. A firm with inelastic demand — perhaps because it has a unique product or strong brand loyalty — can raise prices and increase revenue. Airline pricing illustrates this: business travelers with inelastic demand pay high prices for flexible tickets, while leisure travelers with elastic demand receive deeply discounted fares with restrictions. This is price discrimination based on different elasticities. Brand-name medications face similarly inelastic demand from patients without alternatives, while generic drugs face much more elastic demand because many manufacturers offer the same compound.
Tax Policy and Who Pays
When a government taxes a product, who actually bears the cost depends on the relative elasticities of supply and demand. If demand is inelastic and supply is elastic, buyers bear most of the burden, since they cannot easily reduce purchases. If demand is elastic and supply is inelastic, sellers bear more of the burden, because buyers will sharply cut back if prices rise.
Cigarette Taxes
Cigarette taxes are partly justified because demand for cigarettes is relatively inelastic, especially for addicted adult smokers — taxes raise significant revenue because purchases do not fall much. Elasticity is higher among young people who have not yet become addicted, which is one argument for why high taxes may reduce smoking initiation even if they do little to change established smokers' behavior.
Agricultural Prices and the Farm Problem
Agricultural products tend to have inelastic demand — food is a necessity, and people do not eat vastly more simply because prices fall. When farmers have an exceptional harvest, supply rises and prices fall. Because demand is inelastic, the percentage price drop exceeds the percentage quantity increase, so total revenue to farmers can fall even though production rose. This is the farm problem, and it helps explain why agricultural price supports and crop insurance exist in many countries.
Drug Policy and Minimum Wage
If demand for an illegal drug is inelastic, policies that reduce supply and raise street prices may not substantially reduce use, but may instead increase dealer income and user-driven crime. Policies targeting demand — treatment, prevention, education — may be more effective when demand is inelastic. This is a positive observation about likely consequences, not a complete policy recommendation.
Debates about minimum wage often turn on the elasticity of labor demand. If labor demand is inelastic, a minimum wage increase raises wages with few job losses; if labor demand is elastic, the same increase may cause significantly more job losses. The empirical evidence on this elasticity remains an active area of research, a topic Chapter 15 returns to.
- Two students are arguing about a proposed tax on sugary beverages. Student A says the tax will barely reduce consumption; Student B says consumption will fall a lot. Which student is making a claim about elasticity? What evidence would help settle their argument?
- Explain how differing elasticities between business and leisure travelers allow airlines to charge different prices for similar seats.
- Explain why a good harvest can sometimes hurt farmers financially, using elasticity and total revenue.
Chapter 9 — Back Matter
Chapter Summary
- Sec 9.1Elasticity measures responsiveness — how much one variable changes relative to a change in another — expressed as a ratio of percentage changes, which makes it unit-free and comparable across markets.
- Sec 9.2Price elasticity of demand equals percentage change in quantity demanded divided by percentage change in price. It is reported as an absolute value, and the midpoint method removes the direction problem in the basic formula.
- Sec 9.3Demand is elastic when E_d > 1, inelastic when E_d < 1, and unit elastic when E_d = 1. Perfectly inelastic and perfectly elastic demand represent the theoretical extremes of zero and infinite responsiveness.
- Sec 9.4Demand elasticity depends on the availability of substitutes (the most important factor), whether the good is a necessity or luxury, its share of the buyer's budget, the time horizon for adjustment, and how broadly the market is defined.
- Sec 9.5When demand is elastic, a price increase reduces total revenue. When demand is inelastic, a price increase raises total revenue. When demand is unit elastic, total revenue is unchanged.
- Sec 9.6Price elasticity of supply equals percentage change in quantity supplied divided by percentage change in price, and is always positive because supply curves slope upward.
- Sec 9.7Supply elasticity depends mainly on the time horizon, plus availability of inputs, flexibility of production, ability to hold inventory, and ease of entry for new firms. Supply is generally more elastic in the long run.
- Sec 9.8Income elasticity of demand classifies goods as normal (positive elasticity, further split into necessities and luxuries) or inferior (negative elasticity).
- Sec 9.9Cross-price elasticity of demand classifies the relationship between two goods: positive values indicate substitutes, negative values indicate complements, and values near zero indicate unrelated goods.
- Sec 9.10Elasticity guides business pricing and price discrimination, determines who bears the burden of a tax, explains the farm problem, informs drug policy debates, and shapes the economic analysis of minimum wage laws.
Key Terms Review
elasticity
A measure of how much one variable responds to a change in another variable, expressed as a ratio of percentage changes.
price elasticity of demand
The percentage change in quantity demanded divided by the percentage change in price.
elastic demand
Demand in which the absolute value of price elasticity is greater than 1; quantity demanded responds more than proportionally to price.
inelastic demand
Demand in which the absolute value of price elasticity is less than 1; quantity demanded responds less than proportionally to price.
unit elastic demand
Demand in which the absolute value of price elasticity equals exactly 1.
perfectly inelastic demand
Demand in which quantity demanded does not respond at all to price changes; elasticity equals 0.
perfectly elastic demand
Demand in which buyers purchase any quantity at one price but none at a higher price; elasticity approaches infinity.
midpoint method
A technique for calculating percentage changes using the average of two values as the base, giving the same elasticity regardless of direction.
total revenue
Price multiplied by quantity sold.
price effect
The effect on revenue from receiving a new price on each unit sold.
quantity effect
The effect on revenue from a change in the number of units sold.
price elasticity of supply
The percentage change in quantity supplied divided by the percentage change in price.
elastic supply
Supply in which elasticity is greater than 1; quantity supplied responds more than proportionally to price.
inelastic supply
Supply in which elasticity is less than 1; quantity supplied responds less than proportionally to price.
perfectly inelastic supply
Supply in which quantity supplied does not respond at all to price changes; a vertical supply curve.
income elasticity of demand
The percentage change in quantity demanded divided by the percentage change in income.
normal good
A good for which demand increases when income increases; positive income elasticity.
inferior good
A good for which demand decreases when income increases; negative income elasticity.
luxury
A normal good with income elasticity greater than 1.
necessity
A normal good with income elasticity between 0 and 1.
cross-price elasticity of demand
The percentage change in quantity demanded of one good divided by the percentage change in the price of another good.
substitutes
Goods with positive cross-price elasticity; a price rise in one increases demand for the other.
complements
Goods with negative cross-price elasticity; a price rise in one decreases demand for the other.
Formula Review
All Formulas in One Place
% Change = (New Value − Old Value) ÷ Old Value × 100
E_d = % Change in Quantity Demanded ÷ % Change in Price
E_d (midpoint) = [(Q₂−Q₁) ÷ ((Q₁+Q₂)÷2)] ÷ [(P₂−P₁) ÷ ((P₁+P₂)÷2)]
Total Revenue = Price × Quantity
E_s = % Change in Quantity Supplied ÷ % Change in Price
Income Elasticity = % Change in Quantity Demanded ÷ % Change in Income
Cross-Price Elasticity = % Change in Q. Demanded of Good A ÷ % Change in Price of Good B
Concept Check
- What does a price elasticity of demand equal to 3.0 mean in plain language?
- If demand is inelastic and price rises, what happens to total revenue?
- Why is demand generally more elastic in the long run than in the short run?
- What is the sign of cross-price elasticity for complements? Explain why.
- A good has income elasticity of −0.6. What does this tell you about the good?
- Why is demand for a specific brand of soft drink more elastic than demand for all soft drinks combined?
- What is the most important determinant of price elasticity of demand?
Elasticity Calculation Practice
Show each step of your work.
- The price of a gym membership rises from $40 to $48 per month. Memberships sold fall from 500 to 400. Calculate price elasticity of demand. Is demand elastic, unit elastic, or inelastic?
- The price of organic milk rises from $5.00 to $5.50 per gallon. Quantity demanded falls from 1,200 to 1,140 gallons per week. Calculate price elasticity of demand.
- The price of solar panels falls from $8,000 to $6,000 per system. Quantity demanded rises from 300 to 450 systems per month. Calculate price elasticity of demand.
- The wage for construction workers rises from $25 to $30 per hour. The number of workers willing to work rises from 1,000 to 1,200. Calculate price elasticity of supply.
- Household income rises by 15 percent. Demand for air travel rises by 30 percent. Calculate income elasticity of demand. Classify the good.
- The price of printer ink rises by 10 percent. Demand for printers falls by 8 percent. Calculate cross-price elasticity. Are printers and ink substitutes or complements?
Graph Interpretation
- A demand curve is very steep — almost vertical. Sketch this demand curve. Is demand more likely elastic or inelastic? Explain using the shape of the curve.
- A supply curve is very flat — almost horizontal. Sketch this supply curve. Is supply more likely elastic or inelastic? Explain.
- Draw two demand curves for the same product: one representing the short run, one the long run. Which is steeper? Why?
Real-World Applications
- An airline serves both business travelers and vacation travelers on the same routes. Explain how differing elasticity allows the airline to charge different prices to each group.
- A local grocery store is deciding whether to raise the price of store-brand pasta (many cheap competitors) or store-brand olive oil (only affordable option in the area). Which price increase causes a larger drop in quantity demanded? Which is more likely to raise total revenue?
- Explain why a tax on a good with inelastic demand raises substantial government revenue, using a concrete numerical example of your own design.
Error Analysis
Error 1
"The price of gasoline rose 25 percent last month and quantity demanded fell 5 percent. Since the price went up and quantity went down, demand must be elastic." — Identify the error and explain the correct reasoning.
Error 2
"If the price of apples rises and apple farmers earn more revenue, demand for apples must be inelastic — so apple growers should always try to raise prices." — Identify the error and explain the correct reasoning.
Error 3
"Butter and margarine have positive cross-price elasticity, which means they are complements — people use them together." — Identify the error and explain the correct reasoning.
Error 4
"Since supply always increases when price increases, supply is always elastic." — Identify the error and explain the correct reasoning.
Common Misconceptions
Misconception 1 — Steepness equals inelasticity
Steeper demand curves do indicate more inelastic demand, but the visual slope depends on the units used on the axes. Elasticity is calculated from percentage changes, not from slope directly. Two curves with the same slope can have different elasticities at different points.
Misconception 2 — Needs mean perfectly inelastic demand
Many goods people "need" still have real elasticity. Someone needs transportation but can substitute biking for driving. Someone needs protein but can substitute beans for beef. True perfect inelasticity is extremely rare.
Misconception 3 — Raising prices always hurts sellers
Whether a price increase helps or hurts total revenue depends entirely on elasticity. When demand is inelastic, raising prices increases revenue, and many successful businesses deliberately create inelastic demand through branding and quality.
Misconception 4 — Elasticity is fixed for a product
Elasticity changes over time, across locations, at different income levels, and at different points on the demand curve. The elasticity of gasoline demand differs between the short run and the long run, and even differs at high versus low prices.
Misconception 5 — Cross-price elasticity only applies to obvious pairs
Any two goods can have cross-price elasticity, not just obvious competitors or complements. If streaming movie prices rise enough, some consumers may substitute reading or outdoor activities instead.
Economic Reasoning Questions
- Explain in your own words why the availability of substitutes is the most important determinant of price elasticity of demand. Use an example to illustrate.
- A farmer produces a crop with inelastic demand. Another farmer produces a luxury food with elastic demand. If both farmers' prices fall 20 percent due to a good harvest, which farmer's total revenue falls more? Which should be more worried about bumper crops?
- Some people argue that taxes on goods with inelastic demand are unfair. Explain why someone might make this argument, then explain what a tax supporter might say in response.
- Why is it important to distinguish the short run from the long run when discussing elasticity? Give one demand example and one supply example.
Mixed Review
- Suppose the government imposes a price ceiling below equilibrium on rental housing, and housing demand is inelastic in the short run. Using supply-and-demand reasoning from Chapter 8 and elasticity reasoning from this chapter, explain what happens to quantity demanded, quantity supplied, and the size of the shortage.
- A technological improvement reduces the cost of producing solar panels (Chapter 7), increasing supply. If demand for solar panels is elastic, what happens to total revenue earned by producers? If demand is inelastic, what happens?
- A firm in a perfectly competitive market (Chapter 12) faces a nearly perfectly elastic demand curve at the market price. Explain why, using the concept of substitutes.
- Using elasticity of supply, explain why agricultural prices tend to fluctuate more in the short run than industrial prices, even when both sectors face similar demand changes.
Chapter 9 Checkpoint
Chapter 9 Checkpoint
Elasticity — Complete without assistance; assesses all key skills and concepts from Chapter 9
Section 1 — Vocabulary and Concepts
- Define elasticity in your own words. Why must it be expressed as a ratio of percentage changes rather than raw unit changes?
- Explain the difference between elastic, inelastic, and unit elastic demand. Give one real-world example of each.
- What is the difference between a normal good and an inferior good? Give one example of each.
- Explain the difference between substitutes and complements in terms of the sign of cross-price elasticity.
Section 2 — Calculations
- The price of a movie ticket rises from $12 to $15. Quantity demanded falls from 2,000 to 1,500 per week. Calculate price elasticity of demand and classify it.
- The price of a craft material rises from $10 to $11 per unit. Quantity supplied rises from 500 to 600 units. Calculate price elasticity of supply and classify it.
Section 3 — Revenue Analysis
- A bookstore sells 400 novels per week at $18 each. It raises the price to $20 and sells 340 novels per week. Calculate the change in total revenue. Is demand elastic or inelastic at this point? Explain how you know without calculating E_d directly.
- A firm believes its demand is inelastic and raises its price by 15 percent, but sales fall by 22 percent. What does this outcome reveal about the firm's original belief?
Section 4 — Applied Policy Reasoning
- A government proposes a tax on a good with very inelastic demand and elastic supply. Explain who is likely to bear most of the economic burden of the tax, and why.
- Explain the "farm problem" using elasticity and total revenue. Why can a good harvest sometimes reduce farmers' income rather than increase it?
Section 5 — Error Identification
- A classmate claims: "Demand for insulin is elastic because patients have no choice but to buy it." Identify the error in this statement and explain the correct relationship between necessity and elasticity.
Looking Ahead
Chapter 10: Consumer Choice and Utility at an Introductory Level
Chapter 9 explained how economists measure responsiveness on both sides of the market, and how that responsiveness determines revenue, tax burdens, and the outcomes of public policy. Chapter 10 turns from market-level analysis to individual consumers. It asks: how do consumers actually decide how much of each good to buy? What does it mean to maximize satisfaction within a budget? Chapter 10 introduces utility — economists' term for the satisfaction consumers get from goods and services — and explores why consumers eventually buy less of something as they consume more and more of it. Understanding consumer choice will deepen your understanding of why demand curves slope downward and what drives the elasticities you studied in this chapter.
MGU Library Connection
- Elasticity Formula Sheet — Companion Resource for this chapter
- Chapter 6: Demand and Chapter 7: Supply — review the underlying curves elasticity describes
- Chapter 8: Market Equilibrium — review price ceilings and floors used in the Mixed Review
- Chapter 12: Perfect Competition — develops the perfectly elastic demand case fully
- Chapter 15: Labor Markets, Wages, and Income — returns to minimum wage and labor demand elasticity
- Appendix E: Formula and Calculation Reference — full formula list including elasticity
- MGU Dictionary entries: elasticity, substitutes, complements, normal good, inferior good, total revenue