The best way to understand the mark scheme is to see it applied to a real question in real time.
Below are two complete walkthroughs: a 10-mark Part (a) and a 15-mark Part (b). Both use real past paper questions. Follow the thinking process, not just the final answer.
Walkthrough 1: 10-Mark Question (Part a)
Question:
"Explain how an increase in the price of petrol might affect the market for private cars." [10 marks]
(Real question, SL Paper 1, 2023)
Step 1: Understand what's being asked
- Command term: "Explain" ā need cause-effect chains, not just "price up, demand down"
- Change: Petrol (complement to cars) becomes more expensive
- Market: Private cars (shift in demand, not movement along curve)
- Marks available: 10 ā need theory + diagram + depth
Step 2: Plan your answer structure
ā Define the relationship (complement)
ā” Explain the incentive (higher running costs)
ā¢ Shift the demand curve LEFT
ā£ Explain consequences (lower price, lower quantity sold)
ā¤ Link to broader market effects (dealerships, production)
Step 3: Model answer (full 10 marks)
ā Define the relationship
Petrol and private cars are complementary goodsā they are consumed together. A car is of little use without petrol; an increase in petrol prices makes car ownership more expensive.
ā” Explain consumer incentives
When petrol becomes more expensive, the total cost of car ownership rises (both purchase price + running costs). Consumers perceive cars as less attractive relative to substitutes like public transport or cycling. They reduce their demand.
ā¢ Diagram: Show demand shift
[Draw D to the left as Dā. Mark original equilibrium (Pā, Qā) and new equilibrium (Pā, Qā) where Pā < Pā, Qā < Qā.]
ā£ Consequences for the market
Demand falls ā excess supply ā car dealers cut prices to clear stock ā equilibrium price FALLS and equilibrium quantity FALLS. This reflects the new market reality: fewer people want cars at any given price.
ā¤ Broader supply-side effects
Car manufacturers may reduce production in response to lower demand. This can affect related markets: fewer cars sold means less demand for car insurance, car finance, car repair services. The effect ripples through the economy.
Marking breakdown
Why this scores 9ā10:
- ā Defines complement relationship (1 mark)
- ā Explains cause-effect logic (2 marks)
- ā Diagram shows correct shift + labels (2 marks)
- ā Explains equilibrium change (2 marks)
- ā Extends to secondary effects (2 marks)
Walkthrough 2: 15-Mark Question (Part b)
Question:
"The government has introduced a tax on sugary drinks. Evaluate the effectiveness of this policy in reducing obesity rates." [15 marks]
(Adapted from real past paper, SL Paper 1, 2024)
Understanding the question
- Command term: "Evaluate" ā need balanced judgment, not just "it works" or "it doesn't work"
- Policy: Specific tax (likely indirect/excise tax)
- Goal: Reduce obesity (health policy outcome)
- Effectiveness: Does it work? For whom? To what extent?
Answer structure (15-mark framework)
ā DEFINE + DIAGRAM (3 marks)
"A specific tax on sugary drinks shifts the supply curve left, raising price and reducing quantity consumed. This uses price signals to reduce consumption of a demerit good."
ā” ARGUMENT 1: Why it might work (3 marks)
"If demand is elastic, quantity falls significantly ā fewer calories consumed ā lower obesity. Revenue from the tax can fund health education."
ā¢ ARGUMENT 2: Why it might NOT work (3 marks)
"Demand is inelastic: sugar addiction means consumers keep buying despite higher prices. They shift to other high-calorie drinks (juice, milkshakes) not taxed. Low-income families bear the burden (regressive)."
ā£ EVIDENCE + CONTEXT (3 marks)
"In Mexico, a similar tax reduced sugary drink consumption by 5-6% in first year. But obesity rates barely changed: diet is complex, requires multiple interventions (exercise, nutrition education)."
ā¤ EVALUATION + JUDGMENT (3 marks)
"Effectiveness depends on elasticity AND whether it's part of a broader strategy. As a standalone policy, it's limited. Combined with education and exercise programs, it contributes to a solution. Success varies by income group and cultural context."
Why this approach scores 13ā15 marks
- Acknowledges multiple perspectives (elasticity, equity, behavioral complexity)
- Uses real-world evidence (Mexico case)
- Makes a nuanced judgment ("depends on...")
- Links to broader policy context
- No simplistic conclusions
Common mistakes in walkthroughs
ā Mistake 1: No diagram
"The tax makes drinks more expensive, so people buy less." ā This is description, not explanation. Show the S curve shift, mark Pc, Pp, Qt, DWL.
ā Mistake 2: One-sided evaluation
"The tax is effective because it reduces consumption." ā Where's the other side? What about substitution, regressivity, enforcement costs?
ā Mistake 3: No context
Generic answer about taxes. Use the real example: Mexico, Denmark, US cities. Show you know the evidence.
ā Strength: Conditional reasoning
"It depends on elasticity." ā This shows understanding that economic outcomes vary based on conditions. Top-band answers use conditional language.
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