Cambridge IAL Economics common mistakes
Cambridge IAL Economics · Understand the traps and how to avoid them.
Drawing a supply or demand diagram to show a price ceiling or floor and then labelling the shortage or surplus as the quantity actually traded, rather than reading the traded quantity off the shorter side of the market.
Candidates memorise 'a maximum price causes a shortage' and stop there, so they mark the gap between Qd and Qs as if that gap were the outcome, instead of the constrained quantity min(Qs, Qd) at the ceiling and max(Qs, Qd) at the floor.
What to do instead
After drawing the ceiling or floor, always state explicitly: 'quantity traded is the smaller of quantity demanded and supplied at the controlled price'. Then shade the welfare loss triangle between the controlled quantity and the free-market equilibrium quantity, and label consumer surplus and producer surplus separately so the lost surplus is visible rather than asserted.
Treating an indirect tax and a subsidy as shifting the demand curve, or reversing the direction of the shift when the tax is levied on consumers rather than producers.
Candidates learn one diagram (tax on producer shifts supply up) and apply it mechanically, forgetting that a tax on consumers shifts demand down and a subsidy shifts the relevant curve in the opposite direction to a tax.
What to do instead
Before drawing, write one line: who is legally liable, and does the curve shift up/down/left/right? Practise the four cases (tax on producer, tax on consumer, subsidy to producer, subsidy to consumer) side by side and check that the incidence split depends on the relative elasticities, not on who pays legally.
Computing a country's comparative advantage by comparing absolute output per unit of input rather than opportunity cost, and then concluding the country with larger absolute output should export everything.
Absolute advantage is the intuitive, earlier-learned idea, and opportunity-cost ratios require an extra calculation step that candidates skip under time pressure.
What to do instead
Always convert to opportunity cost before answering: for each good, opportunity cost = units of the other good forgone per unit produced. Show the two ratios explicitly and state which country has the lower opportunity cost in each good. Then add the terms-of-trade range between the two opportunity-cost ratios to check the answer is consistent.
Explaining a currency depreciation by saying 'exports become cheaper so exports rise' without distinguishing the effect on the price of exports in foreign currency from the effect on the volume of exports, and without mentioning the Marshall-Lerner condition.
The chain of reasoning is compressed into a slogan, so the mechanism (price in foreign currency falls, quantity demanded rises, but total export revenue may fall if demand is inelastic) is lost.
What to do instead
Write the chain in full: depreciation lowers the foreign-currency price of exports and raises the domestic-currency price of imports; the effect on the trade balance depends on price elasticities of demand for exports and imports. State the Marshall-Lerner condition (sum of elasticities exceeds one) and the J-curve timing lag explicitly.
Classifying someone as unemployed when they are economically inactive, for example a full-time student, a retiree, or a discouraged worker who has stopped seeking work.
Everyday usage of 'unemployed' means 'not working', whereas the labour-force definition requires being without work, available to start, and actively seeking. Discouraged workers are the classic trap.
What to do instead
Apply a three-part test to every case: without work, available within the relevant period, actively seeking. If any part fails, the person is outside the labour force. When a question involves discouraged workers, note explicitly that they reduce the labour force, so measured unemployment can fall even though the employment situation has worsened.
Treating a rise in the CPI as proof that every household's cost of living has risen by the same percentage, ignoring the weights of the basket and the difference between the retail prices index and a cost-of-living index.
The index is a single headline number, and candidates do not consider that different households consume different baskets, that weights are fixed, and that substitution and quality change bias the measure.
What to do instead
State the basket and weights explicitly, then discuss how a pensioner's basket (energy, food) differs from a student's. Mention substitution bias, quality-change bias, and the fact that a fixed-weight index overstates cost-of-living increases when relative prices change.
Describing demand-pull inflation only as 'too much money chasing too few goods' without linking it to the output gap, and describing cost-push inflation without showing the inward shift of short-run aggregate supply.
Both explanations are learned as phrases, so the diagrammatic mechanism and the policy implications (output rising versus falling) are never connected.
What to do instead
For each cause, draw the diagram and state the effect on both the price level and real output: demand-pull shifts AD right, raising both; cost-push shifts SRAS left, raising prices and lowering output. Then link to policy: demand-pull calls for contractionary policy; cost-push creates a policy conflict between inflation and unemployment.
Analysing a monopoly or oligopoly by asserting 'firms restrict output and raise price' without deriving the profit-maximising output from marginal revenue equals marginal cost, or confusing the demand curve with the marginal revenue curve.
Candidates learn conclusions rather than the optimisation condition, and the marginal revenue curve for a firm facing a downward-sloping demand curve is steeper and lies below demand, which is easily mis-drawn.
What to do instead
Always start from MR = MC to fix output, then read price off the demand curve at that output. For a monopoly, mark the deadweight loss triangle between the competitive output (where P = MC) and the monopoly output. For oligopoly, use game-theory payoffs or the kinked demand curve only after stating the assumption being made.
Evaluating macroeconomic policy by listing advantages and disadvantages in separate paragraphs without stating the context, the time horizon, or the trade-off, so the judgement is a restatement of the previous paragraph.
Evaluation is taught as 'add pros and cons', and candidates do not realise that marks for evaluation come from weighing, prioritising, and identifying conditions under which a conclusion changes.
What to do instead
End every policy answer with an explicit weighing sentence: 'On balance, X is more appropriate than Y in this economy because..., although if the output gap were larger, Y would dominate.' Use the phrases 'depends on', 'in the short run versus long run', and 'the magnitude of the multiplier determines whether...' to force genuine comparison.
Confusing economic growth with economic development, equating a rise in real GDP per capita with an improvement in living standards, and ignoring the composition of growth, income distribution, and environmental costs.
GDP is the single most available measure, so candidates use it as a proxy for welfare without discussing what it omits (non-market activity, inequality, pollution, leisure).
What to do instead
Define growth as an increase in real output and development as a broader improvement in living standards, then evaluate GDP per capita using at least three named limitations (distribution, non-market output, environmental externalities) and one alternative measure such as the Human Development Index or Gini coefficient.