Apply demand and supply analysis to price controls, linked markets and shifting conditions, and evaluate the consequences

What this dot point is asking

SEAB wants you to apply the demand and supply model to real situations: government price controls, markets that are linked through substitution or joint production, and changing market conditions, and then evaluate the consequences. The central insight is that the same simple model, used carefully, explains a huge range of real-world outcomes once you track which curve moves and why.

The answer

Price ceilings (maximum prices)

A price ceiling is a legal maximum price. It only bites if set below the equilibrium. When binding:

• Quantity demanded exceeds quantity supplied, so there is a persistent shortage.
• Because price cannot ration the good, non-price rationing appears: queues, waiting lists, first-come-first-served, or seller favouritism.
• Black markets may develop, with the good resold above the cap.
• In the long run, low returns can reduce supply further, deepening the shortage.

Ceilings are used for equity (affordable rents, basic foods) but trade efficiency for that aim.

Price floors (minimum prices)

A price floor is a legal minimum price. It only bites if set above the equilibrium. When binding:

• Quantity supplied exceeds quantity demanded, so there is a persistent surplus.
• The surplus must be dealt with: stored, bought up by the government, or wasted.
• Examples include minimum wages (a floor in the labour market, where the surplus is unemployment) and agricultural price supports.

Linked markets: substitutes and complements

Markets are connected. A shock in one spills into related markets:

• Substitutes. A rise in the price of coffee (from a supply shock) raises demand for tea, its substitute, raising tea's price and quantity.
• Complements. A fall in the price of cars raises demand for petrol, a complement, raising petrol's price and quantity.

Joint supply and competitive supply

Goods can be linked in production too:

• Joint supply: producing one good automatically produces another (beef and leather). More demand for beef raises leather supply.
• Competitive (or alternative) supply: a producer can make one good or another. A rise in the price of one draws resources away from the other, cutting its supply.

Examples in context

Example 1. Singapore's COE as a quantity control. The Certificate of Entitlement caps the quantity of new cars rather than the price, then lets the market set the premium. Analysing it with demand and supply shows that a fixed quota turns the premium into a market-clearing price that rations the limited certificates to the highest bidders, an example of using the price mechanism to ration a deliberately restricted supply.

Example 2. Minimum wage debates. A minimum wage set above the market-clearing wage is a price floor in the labour market, predicted to create a surplus of labour (unemployment) among low-skilled workers. How large the effect is depends on the elasticity of labour demand, which is why the debate turns on empirical magnitudes, not just the diagram.

Try this

Q1. Explain why a price ceiling causes a shortage only if it is below equilibrium. [2 marks]

• Cue. Below equilibrium the capped price leaves quantity demanded above quantity supplied; at or above equilibrium the ceiling is not binding and has no effect.

Q2. A supply shock raises the price of butter. Explain the effect on the market for margarine. [3 marks]

• Cue. Margarine is a substitute, so the higher butter price raises demand for margarine, shifting its demand curve right and raising margarine's equilibrium price and quantity.

Q3. State two consequences of a binding price floor. [2 marks]

• Cue. A persistent surplus (quantity supplied exceeds quantity demanded), and the need to store, buy up or waste the excess, as with agricultural support or unemployment from a minimum wage.