Difficulty: Introductory to Intermediate | Prerequisites: Supply and demand, equilibrium (see Part 2 study notes).
Markets tend toward equilibrium, but governments often intervene with price controls or quantity restrictions, and the effects are predictable using supply and demand. This section also introduces elasticity, which measures how sensitive buyers (or sellers) are to price changes. Elasticity determines who bears the burden of a tax, whether a price increase raises or lowers a firm's revenue, and how severe the deadweight loss from an intervention will be. These are the tools that turn supply-and-demand diagrams into policy analysis.
Price ceilings (below equilibrium) cause shortages; price floors (above equilibrium) cause surpluses. Both create deadweight loss. Quotas restrict quantity directly. Elasticity tells you whether demand is responsive to price, and that responsiveness determines what happens to total revenue when price changes.
Price ceiling
A legal maximum price set below the equilibrium price. Think of it as a cap: the price cannot go higher than this. Rent control is the classic example.
Price floor
A legal minimum price set above the equilibrium price. Think of it as a platform the price cannot fall below. The minimum wage is the classic example.
Deadweight loss (DWL)
The loss of total surplus that occurs when a market is prevented from reaching equilibrium. In simple terms, it is the value of trades that would have benefited both buyer and seller but no longer happen.
Black market
An illegal market that emerges when price controls create shortages or surpluses. Goods trade at prices above the ceiling (or outside the floor), often with additional risks and costs.
Quota
A government-imposed limit on the quantity of a good that can be traded. Think of it as a cap on quantity rather than price.
Quota rent
The difference between the demand price and the supply price at the quota limit. It represents the profit earned by whoever holds the quota licence.
Price elasticity of demand (Ed)
A measure of how responsive quantity demanded is to a change in price, calculated as the percentage change in quantity demanded divided by the percentage change in price.
Elastic demand
Demand where |Ed| > 1, meaning quantity demanded changes proportionally more than price. Consumers are highly responsive.
Inelastic demand
Demand where |Ed| < 1, meaning quantity demanded changes proportionally less than price. Consumers are not very responsive.
Unit elastic demand
Demand where |Ed| = 1. The percentage change in quantity exactly matches the percentage change in price.
Midpoint method
A way of calculating elasticity that uses the average of the initial and final values as the base, avoiding the problem of getting different answers depending on which direction you measure.
Total revenue
Price multiplied by quantity sold. The relationship between elasticity and total revenue is one of the most practically useful concepts in the course.
A price ceiling is only binding (has an effect) when set below the equilibrium price
Creates a shortage: quantity demanded exceeds quantity supplied at the ceiling price
Consequences:
Inefficiently low quantity traded
Misallocation of goods (goods may not go to those who value them most)
Wasted resources (people queue, search, or bribe to obtain the good)
Reduced quality (sellers cut corners since they cannot raise the price)
Black markets emerge where the good trades at illegal, higher prices
A price floor is only binding when set above the equilibrium price
Creates a surplus: quantity supplied exceeds quantity demanded at the floor price
Consequences:
Excess production that may be destroyed or stockpiled
Inefficient allocation of sales among sellers
Wasted resources on seeking the right to sell
Artificially high quality or over-specification
Both ceilings and floors prevent mutually beneficial trades from taking place
The lost surplus from those blocked trades is the deadweight loss
On a diagram, DWL appears as a triangle between the supply and demand curves, from the controlled quantity to the equilibrium quantity
A quota limits the total quantity of a good that can be traded
At the quota quantity, the demand price (what buyers are willing to pay) exceeds the supply price (what sellers need to be paid), creating a wedge
Quota rent is the difference between these two prices, multiplied by the quota quantity. It represents the profit captured by quota holders.
Like price controls, quotas create deadweight loss by preventing trades that would have occurred at equilibrium
Quotas can lead to illegal activity as people try to trade beyond the limit
Elasticity measures responsiveness: how much does quantity demanded change when price changes?
Calculated as: percentage change in quantity demanded / percentage change in price
The result is typically negative (because demand slopes downward), but we usually work with the absolute value
|Ed| > 1 (elastic): quantity responds more than proportionally to price. A 10% price increase causes more than a 10% drop in quantity.
|Ed| < 1 (inelastic): quantity responds less than proportionally. A 10% price increase causes less than a 10% drop in quantity.
|Ed| = 1 (unit elastic): quantity responds proportionally. The percentage changes are equal.
Standard percentage-change calculations give different answers depending on whether you measure from A to B or B to A
The midpoint method fixes this by using the average of the two values as the base:
%ΔQ = (Q2 – Q1) / ((Q1 + Q2) / 2)
%ΔP = (P2 – P1) / ((P1 + P2) / 2)
This gives a consistent elasticity regardless of direction
Total revenue = Price × Quantity
The relationship between elasticity and total revenue is critical:
Inelastic demand (|Ed| < 1): raising price increases total revenue (the price effect dominates)
Elastic demand (|Ed| > 1): raising price decreases total revenue (the quantity effect dominates)
Unit elastic (|Ed| = 1): total revenue is unchanged by price changes
Elasticity is not constant along a linear demand curve
At high prices and low quantities, demand tends to be elastic
At low prices and high quantities, demand tends to be inelastic
This matters because the revenue implications of a price change depend on where you are on the curve
Price elasticity of demand:
E_d = \frac{\%\Delta Q_d}{\%\Delta P}Midpoint method:
E_d = \frac{(Q_2 - Q_1) \,/\, \frac{Q_1 + Q_2}{2}}{(P_2 - P_1) \,/\, \frac{P_1 + P_2}{2}}Total revenue:
TR = P \times QQuota rent:
\text{Quota rent} = (P_{\text{demand}} - P_{\text{supply}}) \times Q_{\text{quota}}Rent control is a price ceiling. Cities with rent control (New York, Berlin) consistently see housing shortages, reduced maintenance, and black-market subletting.
Minimum wage is a price floor in the labour market. When set above the equilibrium wage, it can cause a surplus of labour (unemployment), though the size of the effect depends on the elasticity of labour demand.
Taxi medallions are a quota. Limiting the number of taxis raises fares (quota rent for medallion holders) and creates deadweight loss. Ride-sharing apps effectively bypassed this quota.
Elasticity and tax policy: governments prefer to tax goods with inelastic demand (cigarettes, petrol) because quantity demanded falls less, so revenue is high and deadweight loss is relatively small.
Students often think a price ceiling always causes a shortage. It only does so when set below equilibrium. A ceiling above equilibrium is non-binding and has no effect.
A common error is calculating elasticity using the starting point as the base. Use the midpoint method unless told otherwise, or you will get two different answers for the same price change.
Students sometimes assume elastic demand means demand is "high." It means demand is responsive to price, not that lots of people want the good.
Many students forget that elasticity varies along a linear demand curve. At the top (high price), demand is elastic; at the bottom (low price), it is inelastic.
⚠️ Price control questions almost always ask you to identify the shortage or surplus, calculate the deadweight loss, and explain the welfare consequences. Draw the diagram.
⚠️ Elasticity-and-revenue questions are extremely common. If |Ed| > 1, a price increase lowers total revenue. If |Ed| < 1, it raises total revenue. Know this cold.
⚠️ Midpoint method calculations appear in problem sets. Practice the formula until it is automatic.
⚠️ Quota rent is tested less often but appears in more advanced problem sets. Be able to identify it on a diagram as the rectangle between the demand and supply prices, with width equal to the quota quantity.
True or false: A price ceiling set above the equilibrium price creates a shortage. (False – it is non-binding and has no effect.)
Fill in the blank: When demand is inelastic, raising the price ______ total revenue. (Increases.)
True or false: The midpoint method gives the same elasticity whether you measure from point A to B or B to A. (True – that is its purpose.)
Fill in the blank: Quota rent is the difference between the ______ price and the ______ price at the quota quantity. (Demand; supply.)
True or false: Deadweight loss from a price floor is zero if the floor is set below equilibrium. (True – the floor is non-binding.)
Q: The government imposes a price ceiling of £3 on a good whose equilibrium price is £5. Using a supply-and-demand diagram, explain the consequences.
A: At £3, quantity demanded exceeds quantity supplied, creating a shortage. The quantity traded falls to whatever sellers are willing to supply at £3 (a point on the supply curve). Deadweight loss appears as a triangle between the supply and demand curves, from the quantity traded under the ceiling to the equilibrium quantity. Black markets may emerge at prices above £3.
Q: The price of a good rises from £10 to £12, and quantity demanded falls from 100 to 80. Calculate the price elasticity of demand using the midpoint method.
A: %ΔQ = (80 – 100) / ((100 + 80) / 2) = –20 / 90 = –22.2%. %ΔP = (12 – 10) / ((10 + 12) / 2) = 2 / 11 = 18.2%. Ed = –22.2% / 18.2% = –1.22. |Ed| = 1.22. Demand is elastic.
Q: A firm faces inelastic demand for its product. Should it raise or lower its price to increase total revenue? Explain.
A: Raise the price. With inelastic demand, the percentage drop in quantity is smaller than the percentage increase in price, so total revenue (P × Q) rises.
Q: Explain the difference between the effects of a price ceiling and a quota that achieve the same quantity traded.
A: Both reduce quantity traded below equilibrium and create deadweight loss. The key difference is the price consumers pay and the distribution of rents. Under a ceiling, the price is capped and any surplus value goes to consumers (or is dissipated in queues and black markets). Under a quota, the price consumers pay rises to the demand price at the quota quantity, and the quota rent (the wedge between demand and supply prices) goes to whoever holds the quota licence.
Q: Why do governments tend to place excise taxes on goods with inelastic demand?
A: When demand is inelastic, a tax causes a relatively small reduction in quantity traded, so tax revenue is high and deadweight loss is low compared with taxing elastic goods. Consumers absorb most of the tax burden because they do not reduce their purchases much.
Elasticity feeds directly into tax incidence analysis (who bears the burden of a tax depends on relative elasticities of supply and demand). Deadweight loss reappears in welfare economics and the analysis of monopoly pricing. Price controls connect to labour economics (minimum wage debates) and housing economics. Quota analysis ties into international trade (import quotas and tariffs).
Price ceiling, price floor, rent control, minimum wage, binding vs non-binding, shortage, surplus, deadweight loss, DWL, welfare loss, black market, quota, quantity control, quota rent, wedge, licence, price elasticity of demand, elastic, inelastic, unit elastic, midpoint method, arc elasticity, total revenue, revenue test, TR and elasticity, percentage change, tax incidence, excise tax, consumer surplus, producer surplus