IB Economics demand & supply explained simply: demand represents consumers’ willingness and ability to buy, supply represents producers’ willingness and ability to sell, and their interaction determines a market’s equilibrium price and quantity. For exams, however, knowing these definitions is not enough. You must distinguish movements from shifts, identify the correct determinant, draw an accurate diagram, and explain the adjustment to a new equilibrium.
Demand, supply, and competitive market equilibrium are separate microeconomics topics in the current IB Economics course. They can be assessed through extended responses, data-response questions and, at HL, quantitative or policy-focused questions. The central exam skill is converting a change described in words into a precise causal chain and correctly labelled diagram.
Demand in IB Economics
Demand is the willingness and ability of consumers to purchase a good or service at different prices over a given period, ceteris paribus. Ceteris paribus means that other relevant factors are assumed to remain constant.
The law of demand states that, ceteris paribus, as the price of a product rises, its quantity demanded falls, and as its price falls, quantity demanded rises. This negative causal relationship produces a downward-sloping demand curve.
It is important to say quantity demanded, rather than demand, when describing a price change. A change in the good’s own price causes movement along the existing curve; it does not shift demand.
Non-price determinants of demand
A demand curve shifts when a non-price determinant changes.
| Determinant | Likely effect on demand |
|---|---|
| Consumer income | Higher income increases demand for a normal good but decreases demand for an inferior good |
| Tastes and preferences | Greater popularity shifts demand right |
| Price of a substitute | A rise in the substitute’s price increases demand for the product |
| Price of a complement | A rise in the complement’s price decreases demand for the product |
| Future price expectations | An expected future price rise may increase current demand |
| Number of consumers | A larger market population generally increases market demand |
For example, if the price of coffee rises, there is a contraction in quantity demanded for coffee along its demand curve. If the price of tea, a substitute, rises, the demand for coffee increases, shifting coffee’s demand curve right.
Supply in IB Economics
Supply is the willingness and ability of producers to offer a good or service for sale at different prices over a given period, ceteris paribus. The law of supply states that a higher price normally causes an increase in quantity supplied, while a lower price causes a decrease in quantity supplied.
This positive relationship is represented by an upward-sloping supply curve. A higher market price creates an incentive for existing firms to expand output because production may become more profitable, assuming other factors remain unchanged.
Non-price determinants of supply
| Determinant | Likely effect on supply |
|---|---|
| Costs of factors of production | Higher wages, energy costs or raw-material prices decrease supply |
| Technology | Productivity-enhancing technology generally increases supply |
| Indirect taxes | Higher taxes raise firms’ costs and shift supply left |
| Subsidies | Subsidies lower firms’ effective costs and shift supply right |
| Number of firms | Entry into the market increases market supply |
| Future price expectations | An expected future price rise may reduce current supply as output is withheld |
| Prices of competitively supplied goods | Producers may switch resources toward the more profitable alternative |
| Joint supply | Producing more of one good may increase the supply of a related by-product |
Avoid saying that higher production costs increase the product’s price directly. The complete model-based explanation is that higher costs decrease supply, shifting the supply curve left; at the new equilibrium, price rises and quantity falls.
Movements Along Curves Versus Shifts
This distinction is among the most frequently tested demand and supply skills.
| Change | Correct result | Diagram action |
|---|---|---|
| Product’s own price changes | Change in quantity demanded or supplied | Move along the existing curve |
| Non-price demand determinant changes | Increase or decrease in demand | Shift the demand curve |
| Non-price supply determinant changes | Increase or decrease in supply | Shift the supply curve |
Use extension and contraction for movements along a curve. Use increase and decrease for shifts of the entire curve.
A common error is to observe that equilibrium price has changed and then shift both curves. The question is not what happened to price, but what caused the original market change. The other curve usually remains fixed unless the scenario gives a separate reason for it to shift.
Competitive Market Equilibrium
Market equilibrium occurs where quantity demanded equals quantity supplied. The intersection of demand and supply identifies the equilibrium price and equilibrium quantity.
At a price above equilibrium, quantity supplied exceeds quantity demanded, creating excess supply, or a surplus. Sellers have an incentive to lower prices, causing quantity demanded to expand and quantity supplied to contract until equilibrium is restored.
At a price below equilibrium, quantity demanded exceeds quantity supplied, creating excess demand, or a shortage. Competition among buyers and the opportunity for sellers to charge more place upward pressure on price. Quantity demanded contracts and quantity supplied expands as the market moves toward equilibrium.
Predicting a new equilibrium
For a single shift, the outcomes should become automatic:
| Market change | Equilibrium price | Equilibrium quantity |
|---|---|---|
| Demand increases | Rises | Rises |
| Demand decreases | Falls | Falls |
| Supply increases | Falls | Rises |
| Supply decreases | Rises | Falls |
If both curves shift, do not guess. An increase in demand and an increase in supply will definitely raise equilibrium quantity, but the effect on price depends on the relative sizes of the shifts. State that the price effect is indeterminate without further information.
Competitive equilibrium is also connected to allocative efficiency. In the basic model, community surplus, consisting of consumer and producer surplus, is maximized where marginal benefit equals marginal cost. Later microeconomics topics examine why externalities, market power, information failures and other conditions can prevent a market outcome from being socially efficient.
How to Draw a High-Quality IB Diagram
A demand and supply diagram should include:
- A vertical axis labelled Price and horizontal axis labelled Quantity
- Curves labelled D and S
- Initial and new curves identified clearly, such as D1 and D2
- Equilibrium points or corresponding prices and quantities
- Arrows showing the direction of a shift where useful
- A title or contextual labels identifying the relevant market
When explaining the diagram, refer to its labels explicitly. A strong response might state: “Higher fertilizer costs increase wheat producers’ costs of production, decreasing supply from S1 to S2. At the original price P1, a shortage develops. Upward pressure on price produces a new equilibrium at P2 and Q2, with a higher price and lower quantity.”
Do not draw a curve shift without explaining the causal mechanism. Likewise, a written explanation does not repair a diagram that shifts the wrong curve.
How Demand and Supply Questions Are Phrased
IB questions use command terms to indicate the required depth. Typical instructions include:
- Define demand, supply or market equilibrium: give a precise meaning.
- Distinguish between a movement and a shift: make the difference explicit and address both concepts.
- Explain using a diagram: provide a causal account and integrate an accurate diagram.
- Calculate equilibrium or excess demand: show the relevant working.
- Discuss or evaluate a market intervention: present balanced analysis and a supported judgment.
A reliable explanation follows this sequence:
- Identify the changed determinant.
- State which curve shifts and in which direction.
- Identify any shortage or surplus at the original price.
- Explain the resulting pressure on price.
- State the new equilibrium price and quantity.
- Refer directly to the diagram.
At HL, students should also be prepared to work with linear functions. If demand is Qd = 100 - 2P and supply is Qs = 20 + 2P, equilibrium occurs where Qd = Qs. Solving 100 - 2P = 20 + 2P gives P = 20, and substitution gives Q = 60.
Common Mistakes That Lose Marks
- Treating a change in price as a shift of demand or supply
- Confusing demand with quantity demanded
- Naming a determinant without explaining how it affects buyers or firms
- Shifting supply right when production costs rise
- Leaving axes, curves, equilibrium values or shifts unlabelled
- Saying a shortage means low demand rather than quantity demanded exceeding quantity supplied
- Claiming a definite result when simultaneous shifts make one variable indeterminate
- Giving a memorized explanation that does not use the market named in the question
Elasticity should not be confused with a curve shift. Price elasticity of demand measures the responsiveness of quantity demanded to a price change along a given demand curve, while price elasticity of supply measures the responsiveness of quantity supplied. Elasticity changes the magnitude of a response, not the movement-versus-shift rule.
Turning Theory Into Exam Marks
Begin with retrieval practice: reproduce the determinant tables and four single-shift equilibrium outcomes from memory. Then practise short scenarios, deciding the curve, direction, price effect and quantity effect before drawing anything.
Move next to complete exam responses. The IB Economics Questionbank lets you isolate demand, supply and competitive equilibrium, while the dedicated demand practice questions, supply determinant questions and market equilibrium questions make weak areas easier to diagnose.
To see the method applied rather than only reading model theory, use the worked and video resources in the RevisionDojo IB Economics hub. Open questions individually and compare each step of the solution with your own identification, diagram and explanation. RevisionDojo’s Economics predicted and mock papers also provide paper-level practice with model answers and available video solutions.
After each attempt, classify the error as content, diagram, causal reasoning, calculation or command-term interpretation. Jojo AI can help check whether your explanation actually connects the determinant to the curve shift and new equilibrium, rather than merely restating the diagram.
Conclusion
Demand and supply questions centre on a small set of testable ideas: precise definitions, the laws of demand and supply, non-price determinants, movements versus shifts, equilibrium adjustment and accurate diagrams. The best answers form an unbroken causal chain from the scenario to the curve shift, shortage or surplus, price adjustment and final equilibrium.
Once those steps are secure, timed practice is more valuable than repeatedly rereading notes. Use RevisionDojo’s Questionbank for focused drills, then study worked video solutions and attempt mock or predicted papers to see how the same model is applied under different command terms.
Sources and referenced URLs
- Official IB Economics programme page
- Official IB Economics SL subject brief
- Official IB Economics HL subject brief
- Official IB diploma sample examination papers
- RevisionDojo IB Economics resources
- RevisionDojo IB Economics Questionbank
- RevisionDojo demand Questionbank
- RevisionDojo non-price determinants of supply questions
- RevisionDojo competitive market equilibrium Questionbank
- RevisionDojo Economics predicted and mock papers