Pancyprian Economics common mistakes
Pancyprian Economics · Understand the traps and how to avoid them.
When writing an answer on demand elasticity, the candidate computes the percentage changes but divides the change in quantity by the change in price without dividing each by its own base value, and reports the raw ratio as the elasticity coefficient.
The formula is memorised as "change over change", and the candidate treats quantity and price as if they were measured in the same units; the idea that elasticity is a unit-free ratio of proportional changes is never internalised, so the base values feel like an optional extra step.
What to do instead
Before calculating, write the formula as (ΔQ/Q₁) ÷ (ΔP/P₁) and explicitly fill in the two denominators. Then run a sanity check: a 10% fall in quantity against a 5% rise in price must give −2, not −0.5 or a bare 5. Practise with one example where price and quantity are in very different units (e.g. litres and euros) to make the point that only percentages can be compared.
The candidate labels a good as price-inelastic or price-elastic without distinguishing along which part of a linear demand curve the price change occurs, and states a single elasticity value for the whole curve.
Textbook diagrams show one curve and one coefficient, so the candidate assumes elasticity is a property of the good rather than of the point on the curve; the geometry (elastic above the midpoint, inelastic below) is learned as a label, not as a relationship between price level and revenue.
What to do instead
For every linear-demand question, mark the midpoint first and locate the price change relative to it. Link the answer to total revenue: if a price cut raises revenue, demand was elastic at that point. Do three quick numerical checks on the same curve at high, middle and low prices to see the coefficient change sign of magnitude.
In cost and revenue questions the candidate confuses a shift of a cost curve with a movement along it, for example treating a rise in the wage rate as a movement along the marginal cost curve, or drawing a fall in fixed costs as a downward shift of the marginal cost curve.
The candidate has learned "higher cost means curve moves up" as a reflex without asking which variable is on each axis; because average and marginal cost curves are usually drawn against output, any cost change is drawn as a vertical shift, and the distinction between a change in output and a change in the conditions of production is lost.
What to do instead
Adopt a two-question routine: (1) which variable changed - output or a cost condition? (2) does that variable appear on the horizontal axis? If output changed, move along; if a cost condition changed, shift. Then state explicitly which curves shift: a wage rise shifts MC, AVC and ATC but leaves AFC unchanged; a rise in fixed cost shifts AFC and ATC only.
When asked to compare market structures, the candidate asserts that a perfectly competitive firm or a monopolistically competitive firm earns abnormal profit in long-run equilibrium, or that a monopolist produces at the minimum point of the average cost curve.
Short-run diagrams are over-learned and the long-run adjustment mechanism (entry and exit) is recited as a phrase rather than applied; the candidate also carries the efficient-scale result from perfect competition into other structures without checking whether free entry or a downward-sloping demand curve is present.
What to do instead
For each structure, write the long-run equilibrium condition as an equation before drawing: perfect competition P = MC = minimum ATC; monopoly MR = MC with P above MC and no minimum-ATC condition; monopolistic competition MR = MC with P = ATC but tangency above minimum ATC. Then explain in one sentence why free entry forces the tangency, and check that the diagram matches the equation.
The candidate answers a market-failure question by naming the failure (e.g. negative externality, public good, asymmetric information) and then recommends "government should intervene" or "tax it" without specifying the instrument, the incidence, or the size of the corrective measure.
Mark schemes reward evaluation, but candidates treat the label as the answer; they have not practised linking a diagram (marginal social cost above marginal private cost) to a specific Pigouvian tax equal to the marginal external cost at the socially optimal output.
What to do instead
Use a fixed three-step template: identify the divergence between private and social cost or benefit, state the instrument and its rate or quantity, then evaluate one strength and one limitation (e.g. a tax needs the regulator to know the marginal external cost; tradable permits need an initial allocation). Always mark the socially optimal output on the diagram and tie the tax to the vertical gap at that output.
In national income questions the candidate adds transfer payments, second-hand sales or intermediate goods into GDP, or treats a rise in nominal GDP as a rise in real output without deflating.
GDP is memorised as "total spending in the economy", and the expenditure circular flow diagram makes all spending look equivalent; the candidate has not internalised the production boundary and the distinction between the value of output and the value of transactions.
What to do instead
Apply the value-added test to every item: does this transaction create new domestic output this year? Pensions, unemployment benefit and resale of an existing house fail it; the estate agent's commission passes it. For real versus nominal, always divide by the price index (or use the deflator) and state the base year before comparing two years.
The candidate describes an expansionary monetary policy as "the central bank increases the money supply" and stops there, without naming the instrument (open market operations, the policy rate, reserve requirements) or the transmission channel to spending and prices.
The money multiplier and the tools are learned as separate lists, so the candidate cannot connect a purchase of securities to bank reserves, to lending, to investment and to aggregate demand; the mechanism is recited rather than reasoned.
What to do instead
Write the chain explicitly every time: policy rate cut → lower market rates and higher reserves → credit and asset prices → investment and consumption → aggregate demand → output and price level, with the lags and the liquidity-trap caveat noted. Name the instrument first, then the chain, then one reason the chain may break.
When analysing a budget deficit or the national debt, the candidate treats the deficit as automatically inflationary or automatically crowding out, and mixes the cyclical and structural components of the balance.
The identity between government borrowing and the deficit is learned, but the financing route (bond sales to the public versus purchase by the central bank) and the state of the output gap are not brought into the answer; the candidate also confuses the deficit (a flow) with the debt (a stock).
What to do instead
State the financing assumption before evaluating: borrowing from the private sector can raise interest rates and crowd out investment; monetisation can be inflationary. Separate the cyclical deficit (from the automatic stabilisers at the current output gap) from the structural balance, and always label deficit as a flow per year and debt as a stock at a point in time.
In balance-of-payments questions the candidate adds the current account and the capital and financial account to describe the balance of payments as a whole, or claims that a current account deficit means the country is not competitive.
The accounts are memorised as separate tables and the accounting identity (the sum of the accounts is zero, with errors and omissions) is not applied; the candidate also imports the everyday meaning of "deficit" as a failure into an accounting statement that must balance.
What to do instead
Write the identity first: current account + capital account + financial account + errors and omissions = 0, then explain that a current account deficit is matched by a net capital inflow. Distinguish the causes of a deficit (income growth raising imports, real exchange rate appreciation, weak productivity) before judging it, and note that a deficit may reflect either weakness or strong investment inflows.
When asked about depreciation of a currency, the candidate states that a depreciation always improves the trade balance and always raises domestic inflation, without referring to the Marshall–Lerner condition, the J-curve or the price of imported inputs.
The direction of the effect is learned as a slogan; the elasticity conditions and the timing of the adjustment are treated as optional extras, and the candidate does not separate the volume effect from the price effect on the value of trade.
What to do instead
Condition every answer: depreciation improves the trade balance only if the sum of the import and export demand elasticities exceeds one (Marshall–Lerner), and even then the J-curve means the balance may worsen first as import prices rise before volumes adjust. Add the inflation channel through imported inputs and the pass-through to consumer prices, so the answer covers both the external and the domestic effect.