What it covers: Elasticity, consumer and producer surplus, market efficiency, price controls, and excise taxes/subsidies.
Exam weight: About 20โ25% of the AP Microeconomics exam โ the most heavily weighted unit.
The big question: How do we measure responsiveness in markets, and what happens to market efficiency when government intervenes with price controls or taxes?
Recurring themes: Elasticity drives tax incidence; total surplus is maximized at equilibrium; any policy that moves quantity away from equilibrium creates deadweight loss.
Key topics at a glance
Price Elasticity of Demand
%ฮQd รท %ฮP. Elastic (>1): substitutes available, luxury goods, long time horizon. Inelastic (<1): few substitutes, necessities, short time horizon.
Total Revenue Test
If demand is elastic, a price increase lowers total revenue. If demand is inelastic, a price increase raises total revenue.
Cross-Price & Income Elasticity
Positive cross-price elasticity = substitutes. Negative = complements. Positive income elasticity = normal good. Negative = inferior good.
Price Elasticity of Supply
%ฮQs รท %ฮP. More elastic with more time to adjust production and more available inputs.
Consumer & Producer Surplus
CS = area below demand, above price. PS = area above supply, below price. Total surplus is maximized at equilibrium.
Price Ceilings & Floors
A binding ceiling (below equilibrium) causes a shortage. A binding floor (above equilibrium) causes a surplus. Both create deadweight loss.
Excise Taxes
Shifts supply up by the tax amount. Creates a wedge between price paid and price received, reduces quantity, and creates deadweight loss.
Tax Incidence
The more inelastic side of the market (buyers or sellers) bears more of the tax burden, regardless of who physically pays the tax.
The key terms you must know
Price elasticity of demand/supply โ measures responsiveness of quantity to a price change.
Elastic / inelastic / unit elastic โ coefficient >1, <1, or =1 in absolute value.
Cross-price elasticity โ responsiveness of demand for one good to another good's price (sign reveals substitute vs. complement).
Income elasticity โ responsiveness of demand to income (sign reveals normal vs. inferior good).
Consumer surplus / producer surplus โ the gains buyers and sellers get from trading at the market price.
Allocative efficiency โ total surplus is maximized; occurs at equilibrium, where marginal benefit equals marginal cost.
Price ceiling / price floor โ government-set maximum/minimum price that creates shortage/surplus when binding.
Deadweight loss โ lost total surplus from producing away from the efficient quantity.
Tax incidence โ who actually bears the burden of a tax, determined by relative elasticity.
Key themes to remember
Elasticity is about percentages, not raw amounts. A $1 price change means something very different for a $2 good than a $200 good โ elasticity standardizes for that.
Total surplus is the benchmark for efficiency. Anything that moves the market away from the competitive equilibrium quantity reduces total surplus.
Whoever is less elastic pays more of the tax. This single rule answers most tax incidence questions on the exam.
"Binding" matters. A price ceiling above equilibrium or a price floor below equilibrium has no effect โ only binding controls create shortages, surpluses, and deadweight loss.
Graphing is the skill being tested. Almost every concept in this unit is best explained โ and best answered on the exam โ with a labeled supply-and-demand graph.
Common exam traps
Don't confuse elastic with "big change." Elasticity is a ratio of percentage changes, not the size of the change in dollars.
Total revenue and total surplus are different things. Total revenue is price ร quantity earned by sellers; total surplus includes both consumer and producer gains.
Statutory incidence โ economic incidence. Who is legally required to pay a tax (statutory) is not necessarily who actually bears the cost (economic) โ that depends on elasticity.
A non-binding price control does nothing. Always check whether the ceiling/floor is actually above or below equilibrium before describing any effect.
Deadweight loss is a triangle, not a rectangle. Don't confuse it with tax revenue (a rectangle) or with the transfer between consumer and producer surplus.