Market failure is where the price mechanism, left alone, sends the wrong quantity to market — too little of a merit good, too much of a demerit one. Government intervention nudges it back toward the social optimum, and IB Paper 1 rewards you for drawing the gap and evaluating the tool that closes it. This page sets two tools side by side — a subsidy and a price floor — the way an IB Economics tutor in London would before a mock: one market, two very different side effects.
1 · Market failure, the IB way
An examiner wants the definition first. Market failure is a free market’s failure to allocate resources at the socially optimal quantity, where marginal social benefit meets marginal social cost. Land anywhere else and allocative efficiency is lost; the money value of that loss is a welfare loss you shade, not derive.
Two categories dominate Paper 1. A merit good — vaccination, fresh food — is under-consumed, its social benefit running ahead of the private benefit the buyer weighs; a demerit good is over-consumed for the mirror reason. Behind both sit externalities: spillover costs or benefits on third parties. Your job is to show the gap and explain its direction; no calculus.
2 · Four tools, one diagram
A government reaching into a failing market has four standard tools, and the exam expects you to tell them apart.
- Indirect taxes raise the price of a demerit good to cut consumption.
- Subsidies lower the price of a merit good to raise it.
- Price controls — a floor or ceiling — hold price away from equilibrium.
- Regulation bans, caps, or mandates behaviour directly, without touching price.
The two that push price opposite ways — the subsidy and the floor — carry the rest of this page.
3 · A subsidy on a merit good
A subsidy is a per-unit payment from the government to producers, drawn as a downward shift of supply by its size, s. The price consumers pay falls to Pc, quantity rises to Q1, and the price producers receive rises to Pp, with Pp − Pc = s at Q1. Over every unit sold that wedge is the government’s bill: cost = s × Q1, the rectangle your diagram shades. For a merit good the extra units are the point: they carry consumption toward the optimum the free market missed.
4 · A price floor
A price floor is a legal minimum price, set above equilibrium to support producers’ incomes; below equilibrium it does nothing — a floor bites only from above (Pf > P*). When it binds, quantity supplied Qs jumps while quantity demanded Qd shrinks, and the gap Qs − Qd is an excess supply — a glut. Only the short side trades, so Qd changes hands, and the triangle between demand and supply from Qd back to equilibrium is the welfare loss. The glut then rots, is dumped, or is bought by the government at Pf × (Qs − Qd).
5 · Evaluating like an examiner
The calculation is AO2; the marks that lift a 5 to a 7 are AO3 — discuss, evaluate, to what extent. Go stakeholder by stakeholder. Consumers of the merit good win on a lower price; taxpayers who never buy it fund the bill. Producers gain from both tools, but a floor rewards them for output nobody buys. The government trades a budget cost for a policy goal, and a floor can cost twice: to buy the glut, then to store it. Society gains only if the correction beats its cost. In the short run a subsidy lifts consumption at once; in the long run it can breed dependence.
Flag the information problem: the right subsidy needs the size of the spillover benefit, which no government observes exactly; too high over-corrects, too low leaves the failure. This anchors Paper 1 essays (10 and 15 marks), Paper 2 data response, and HL Paper 3 calculation. On the example, examiners want a named, current policy — deploy it in four moves: intervention type, market, intended effect, one line of evaluation. Bank two or three: agricultural price floors, renewable-energy subsidies, minimum unit pricing for alcohol. Invent nothing.
Worked example — a subsidy and a floor in the market for fresh produce
Take the market for fresh fruit and vegetables — a merit good, under-consumed when buyers weigh only their own benefit. P is in £ per basket, Q in thousands of baskets. Demand P = 22 − Q, supply P = 6 + Q.
Step 1 — Equilibrium. 22 − Q = 6 + Q gives Q* = 8, P* = £14. Both interventions start here.
Step 2 — Apply a subsidy. The government pays s = £4 a basket; supply shifts down to P = 2 + Q.
Step 3 — New equilibrium and the wedge. 22 − Q = 2 + Q gives Q1 = 10 and Pc = £12. Producers receive that £12 plus the £4 subsidy — Pp = £16, exactly the original supply at Q = 10 — so Pp − Pc = £4 = s.
Step 4 — Cost to government. s × Q1 = £4 × 10,000 = £40,000; the extra 2,000 baskets pull consumption toward the optimum.
Step 5 — A floor instead. Return to equilibrium and set Pf = £18 (above £14, so it binds): Qd = 4, Qs = 12, an excess supply of 8,000 baskets, of which only 4,000 sell.
Step 6 — The welfare loss. The baskets between Qd = 4 and Q* = 8 stop trading: ½ × (8 − 4) × (18 − 10) = 16, the triangle your diagram shades. Buying the glut instead would cost the government £18 × 8,000 = £144,000.
Step 7 — The contrast. Same market, opposite effects: the subsidy raised quantity to correct under-consumption; the floor cut it to prop producers up.
Given a failing market and two tools — a subsidy and a price floor — could you draw both cleanly, calculate the government’s cost and the welfare loss, and evaluate who gains and who pays? Building the diagrams from a blank page and matching the tool to the failure, rather than listing the four interventions, is exactly what IB Paper 1 rewards. A one-on-one IB Economics tutor works the subsidy and the floor with you on the command terms the mark scheme uses, until the labelled diagram, the area calculation and the AO3 evaluation are things you produce under exam pressure. Book a trial session.
Practice
Q1 (HL). The market for rooftop solar installations, a merit good, has demand P = 40 − Q and supply P = 10 + Q (P in £, Q in thousands). A subsidy shifts supply to P = 4 + Q. (a) Find the new quantity and the prices consumers pay and producers receive. (b) Calculate the cost to the government from the diagram. (c) State the welfare loss the subsidy would cause if the good were not a merit good.
Q2. A market has demand P = 28 − Q and supply P = 4 + Q, with a price floor at £22. Find Qd and Qs, the excess supply, the welfare loss, and the cost of the government buying the whole surplus.
Answers. Q1: (a) 40 − Q = 4 + Q → Q1 = 18; Pc = £22, Pp = £28, so s = £6. (b) cost = s × Q1 = 6 × 18 = £108,000. (c) the free market makes Q* = 15, so output rises by 3; loss = ½ × s × ΔQ = ½ × 6 × 3 = 9. Q2: the market clears at (12, £16), so £22 binds; Qd = 6, Qs = 18, excess supply 12; welfare loss = ½ × 6 × 12 = 36; buying the surplus costs £22 × 12,000 = £264,000.
Key takeaways
- Market failure is the gap between the market quantity and the socially optimal one — a welfare loss you show, not derive.
- A subsidy shifts supply down by s: price falls, quantity rises, the government pays s × Q1, and Pp − Pc = s.
- A binding price floor sits above equilibrium and opens an excess supply; only the short side trades, and the lost trades are a welfare loss.
- Match the tool to the failure: the subsidy expands a merit good toward its optimum; the floor holds a market away from it, and IB markers reward the labelled diagram and a named current example for both.
Why London IB students choose our IB Economics tutoring
- Command terms, drilled: sessions rehearse the AO1–AO2 verbs — define, explain, draw, calculate — and the AO3 evaluators — discuss, evaluate, to what extent — so you know what each mark asks for.
- Diagram under exam pressure: one-on-one work builds the subsidy and price-floor diagrams from a blank page, correctly labelled — IB markers reward a clean one and dock a sloppy one.
- Real-world examples that land: a tutor works up two or three current interventions with you and drills them, so a Paper 1 essay carries the evidence the mark scheme wants.
FAQ
Q: How is a subsidy shown on a diagram?
A: As a downward shift of supply by the subsidy. The new equilibrium gives the consumer price, and adding the subsidy back gives the producer price; that gap times the new quantity is the government’s cost.
Q: Why does a price floor create a surplus?
A: Because it sits above the equilibrium price. At that higher price more is supplied and less demanded, so quantity supplied exceeds quantity demanded — and only the demanded quantity trades.
Q: What is a merit good in IB Economics?
A: A good under-consumed in a free market because its benefit to society exceeds the buyer’s private benefit. Health care and education are the standard examples, and a subsidy is the usual correction.
Q: Do I have to calculate welfare loss for IB?
A: You show it more often than you derive it. Higher Level may ask you to calculate an area — a subsidy’s cost or a welfare-loss triangle — with ½ × base × height. No calculus at either level.
Q: Which papers test market failure and intervention?
A: Paper 1 essays at 10 and 15 marks, where a diagram and a real-world example score; Paper 2 as data response; and HL Paper 3, which asks for the calculation.
Work with an IB Economics tutor in London or online
Market failure and intervention is a Paper 1 favourite, and it rewards the student who can draw the subsidy and the floor cleanly, calculate the cost and the welfare loss, and back it with a current real-world example. One-on-one sessions build exactly that, on the command terms the IB actually uses. Tell us your exam session and whether you sit Standard or Higher Level, and we will match you with the right tutor this week — in person or fully online.
