NSSCAS Economics (8246) common mistakes
NSSCAS Economics (8246) · Understand the traps and how to avoid them.
When asked to explain the basic economic problem, candidates describe 'scarcity' only as 'things are limited' and then list examples of money shortages or poverty rather than explaining the mismatch between unlimited wants and finite resources, and they omit the need for choice and opportunity cost.
Textbook definitions get memorised as a single phrase and never connected to the three questions (what, how, for whom). Candidates conflate scarcity with poverty because everyday language uses 'scarce' to mean 'hard to get'.
What to do instead
Practise writing the chain in one sentence: wants are unlimited, resources are finite and have alternative uses, therefore choices must be made, and every choice has an opportunity cost (the next best alternative forgone). Then apply the chain to a specific example such as allocating a government budget between a hospital and a road, naming the opportunity cost explicitly.
On price elasticity of demand questions, candidates calculate the coefficient correctly but then interpret it wrongly: they say a PED of -0.4 means demand is elastic because '0.4 is a big number', or they ignore the sign and state the good is 'inelastic and therefore a luxury'.
The formula is learned mechanically without linking the magnitude to the responsiveness of quantity to price. The negative sign convention and the revenue implications are treated as separate facts rather than consequences of the same idea.
What to do instead
Always finish a calculation with a sentence of the form: 'A 1% rise in price causes a 0.4% fall in quantity demanded, so demand is price-inelastic; total revenue therefore rises when price rises.' Use the total revenue test as a cross-check: if price and revenue move the same way, demand is inelastic.
When drawing or describing market diagrams, candidates shift the demand curve when the cause is a change in the good's own price, or they move along the curve when the cause is a change in income, tastes or the price of a substitute. They also fail to label axes, curves and the original and new equilibrium.
The distinction between a movement along a curve and a shift of a curve is a diagrammatic convention that is easy to state but hard to apply under time pressure, and candidates rush the diagram to get to the written explanation.
What to do instead
Before drawing, classify the cause in one line: 'own price changed → movement along; any other determinant changed → shift'. Use a consistent diagram template with labelled axes (Price, Quantity), labelled curves (D, D1, S, S1), and marked E and E1 with dotted lines to the axes. Practise three examples per determinant until the classification is automatic.
In market failure questions, candidates assert that a negative externality 'means the market fails' and stop there, without identifying the divergence between marginal social cost and marginal private cost, without stating whether the good is over- or under-produced, and without explaining the welfare loss triangle.
Externality questions are answered with a definition and a diagram copied from memory rather than with a causal argument linking private decision-making to the social outcome.
What to do instead
Use a fixed four-step structure: (1) define the externality and name the third party affected; (2) show MSC > MPC (or MSB > MPB) with a diagram; (3) state that the free market output exceeds (or falls short of) the socially optimum because private agents ignore the external cost; (4) identify the deadweight loss area and then evaluate a policy against it.
When evaluating government intervention, candidates list policies (tax, subsidy, regulation, tradable permits) without linking each to the specific market failure, and they claim a tax 'solves' the externality without discussing the difficulty of valuing the external cost, inelastic demand, or regressive effects.
Evaluation is treated as a separate 'add-on' paragraph of generic advantages and disadvantages rather than as an argument about whether this instrument corrects this divergence in this market.
What to do instead
For each policy, write one sentence of mechanism ('a specific tax equal to the marginal external cost shifts MPC to MSC and reduces output to the social optimum') followed by one sentence of limitation tied to the same mechanism ('but the regulator cannot observe MEC precisely, so the tax may be too high or too low'). Aim for two policies with mechanism-plus-limitation each, then a short judgement.
On cost and revenue questions, candidates confuse the shape and cause of short-run and long-run average cost curves, assert that the law of diminishing returns explains the whole U-shaped LRAC, and mix up fixed and variable costs when calculating total, average and marginal cost.
The short-run (fixed capital, diminishing marginal returns) and long-run (all factors variable, economies and diseconomies of scale) stories are learned as two diagrams rather than as two different assumptions about which inputs can change.
What to do instead
Anchor each curve to its assumption: 'in the short run at least one factor is fixed, so diminishing marginal returns make MC rise and pull AC up; in the long run all factors vary, so LRAC is U-shaped because of economies then diseconomies of scale.' Do a numeric drill converting a table of output, TFC and TVC into TC, AFC, AVC, AC and MC, and check MC crosses AC at AC's minimum.
In market structure comparisons, candidates describe monopoly and perfect competition by listing features ('one firm', 'many firms') without linking structure to conduct and performance, and they claim a monopolist always charges the highest possible price or always earns supernormal profit in the long run.
Features are memorised as a checklist rather than derived from the underlying assumptions about barriers to entry, product differentiation and information, so the consequences for efficiency and price are not connected.
What to do instead
For each structure, write the chain: assumptions → conduct (price-setting power, output choice where MR = MC) → performance (allocative and productive efficiency, supernormal profit persistence). Explicitly note that a monopolist is constrained by the demand curve and that supernormal profit persists only because barriers to entry block the competitive process.
When calculating national income or the multiplier, candidates mix up GDP and GNP, include transfer payments and the sale of second-hand goods in GDP, and apply the multiplier to a change in injections without using the correct marginal propensity to withdraw.
The circular flow is learned as a diagram but the accounting rules (what counts as production in the period, what is a transfer) are not practised, and the multiplier formula is recalled without its components.
What to do instead
Keep a short inclusion/exclusion list: include newly produced final goods and services in the period; exclude transfers, second-hand sales, intermediate goods and purely financial transactions. For the multiplier, write k = 1/(1 − MPC) = 1/(MPS + MPT + MPM) and state which withdrawals are assumed constant before applying it.
On inflation and monetary policy questions, candidates confuse causes (demand-pull versus cost-push) with effects, describe raising interest rates as 'reducing inflation' without the transmission chain, and mix up the policies of the central bank with those of the government.
The transmission mechanism (interest rate → borrowing and saving → consumption and investment → aggregate demand → price level) is a multi-step chain that is compressed into a single assertion under exam pressure, and institutional roles are blurred.
What to do instead
Write the chain in numbered steps every time: policy rate up → cost of borrowing up and return on saving up → consumption and investment fall → AD shifts left → demand-pull pressure and the price level fall, with a time lag. Add one limitation (inelastic investment, exchange rate appreciation hurting exports, or cost-push inflation being unresponsive) and keep central bank and treasury roles explicitly separate.
On fiscal policy and the balance of payments, candidates describe a budget deficit as automatically 'bad', ignore the distinction between cyclical and structural deficits, and treat a current account deficit as a loss of competitiveness without considering the causes (strong currency, income growth, investment income flows) or the financing through the financial account.
Everyday political language ('deficit is bad', 'trade deficit is losing') overrides the analytical framework, and the balance of payments identity (current account plus financial account plus errors and omissions equals zero) is not used to structure the answer.
What to do instead
Start from the identity and the stage of the cycle: a deficit may be cyclical (recession lowering tax receipts), structural, or the mirror of capital inflows financing investment. For the current account, classify the cause first (price competitiveness, exchange rate, relative income growth, investment income), then assess whether it is a problem, and always state how it is financed.