AP Intermediate Economics (BIEAP) common mistakes
AP Intermediate Economics (BIEAP) · Understand the traps and how to avoid them.
Treating 'increase in quantity demanded' and 'increase in demand' as interchangeable, so a price fall is described as shifting the demand curve rightward.
Textbook language is loose in casual reading, and students memorize 'price up, demand down' without attaching it to a movement along a curve versus a shift of the curve. The graph looks the same in their mind either way.
What to do instead
Before answering, ask: did a non-price determinant (income, tastes, prices of related goods, expectations, number of buyers) change? If yes, shift the curve and call it increase/decrease in demand. If only own-price changed, move along the curve and call it increase/decrease in quantity demanded. Practise by classifying ten one-line scenarios aloud into the two buckets.
Computing elasticity of demand as a plain percentage ratio without the midpoint (arc) formula, then labelling the good elastic or inelastic from a single point estimate.
The simple formula is taught first and the base-value problem (different results for price rises versus falls) is easy to miss under time pressure. Students also forget that elasticity changes along a straight-line demand curve.
What to do instead
Use the arc formula with average price and average quantity as denominators unless the question says 'point elasticity'. State the classification rule explicitly: |E| > 1 elastic, = 1 unitary, < 1 inelastic, 0 perfectly inelastic, infinity perfectly elastic. When a question gives a linear demand function, compute elasticity at the stated price only and note that it differs at other prices.
Mixing up the laws of returns to a factor (short run, one input variable) with returns to scale (long run, all inputs variable), and answering a returns-to-scale question with diminishing marginal product reasoning.
Both are called 'laws of returns', both produce U-shaped or falling curves, and the short-run case is drilled far more heavily, so it becomes the default answer.
What to do instead
Anchor on the time horizon: is any input fixed? If yes, it is returns to a factor and the answer is increasing/diminishing/negative marginal returns. If all inputs change proportionally, it is returns to scale and the answer is increasing/constant/decreasing. Write the two headings side by side in your notes with one worked numerical example each before the exam.
Confusing the short-run shutdown point with the break-even point, and concluding a firm should exit whenever price falls below average total cost.
Both involve a price-cost comparison and both sit on the marginal cost curve. Students remember 'below ATC means loss' but forget that in the short run fixed costs are sunk, so producing at a loss can still minimise loss.
What to do instead
Memorise the decision rule precisely: shut down in the short run only if price falls below minimum average variable cost; between min AVC and min ATC, produce where P = MC because revenue covers variable cost and part of fixed cost; break-even is at min ATC. Redraw the three-region diagram from memory and label the shutdown and break-even points.
Describing a monopoly as charging 'the highest possible price' rather than the price the market will bear at the profit-maximising output where marginal revenue equals marginal cost.
Everyday usage of 'monopoly power' suggests unlimited pricing, and the profit-maximising condition is learned as a formula without the accompanying point on the demand curve.
What to do instead
Work the two-step routine every time: set MR = MC to get output, then read the price off the demand curve at that output. Note that a monopolist never operates on the inelastic portion of its demand curve, because there MR is negative. Compare with perfect competition by stating price equals MC there but exceeds MC under monopoly.
Treating rent, wages, interest and profit as arbitrary payments rather than as prices of factor services determined by derived demand and supply, and therefore failing to explain why a factor's price equals its marginal revenue product under competition.
Factor pricing is taught late and briefly, so students fall back on descriptive answers ('workers are paid for their labour') instead of the marginal productivity logic used in the rest of the paper.
What to do instead
Learn the single competitive result MRP = MRC = factor price, and be able to derive the downward-sloping factor demand curve from diminishing marginal product and falling output price. For each factor, name the supply curve's shape and what shifts it. Add one line on transfer earnings and economic rent to cover the surplus element.
Adding transfer payments, second-hand sales, intermediate goods and purely financial transactions into national income, or double counting the value of output instead of value added.
These flows look like income to the people receiving them, and the expenditure and income methods are learned separately, so the exclusion rules are not applied consistently across methods.
What to do instead
Keep a one-line exclusion list: intermediate goods, second-hand goods, transfer payments, capital gains and share purchases, and output produced abroad by residents versus at home by non-residents. Then check the identity: value added by all firms equals factor incomes equals final expenditure, and confirm your figure reconciles across at least two methods before writing it down.
Calculating the money multiplier as 1 divided by the cash reserve ratio while ignoring currency held by the public, and treating the central bank's policy rate changes as automatically changing the money supply.
The simple deposit multiplier is the version most often memorised, and the link between repo rate, bank reserves and broad money involves several steps that get compressed into a slogan.
What to do instead
State which multiplier you are using: the simple deposit multiplier 1/CRR for a single bank's deposit expansion, or the broader money multiplier (1 + c)/(c + r) where c is the currency-deposit ratio. Then trace the transmission chain explicitly: policy rate to reserve cost to lending rates to credit to deposits to money supply, noting lags and that banks may hold excess reserves.
Treating a government budget deficit as identical to public debt, and answering a question about fiscal stance with a definition of debt rather than the change in the deficit.
The words are used interchangeably in news coverage, and the stock-versus-flow distinction is rarely tested explicitly in class, so it goes unnoticed until an exam question hinges on it.
What to do instead
Fix the stock-flow pair: deficit is a flow over a year, debt is the accumulated stock of past borrowing. Separate revenue deficit, fiscal deficit and primary deficit, and remember primary deficit equals fiscal deficit minus interest payments. When asked about fiscal stance, quote the deficit measure and whether it is rising or falling, then mention debt only as the consequence.
Reading the current account and capital account of the balance of payments as two alternative measures of the same thing, and calling a current account deficit a balance of payments deficit.
Both are called 'the balance of payments', the accounting identity that they must sum to zero with reserves is counterintuitive, and students conflate a trade deficit with an overall external imbalance.
What to do instead
Draw the ledger: current account covers goods, services, primary income and transfers; capital and financial account covers investment flows and reserve changes; the two must offset, so an overall surplus or deficit shows up as reserve accumulation or drawdown. Practise interpreting a current account deficit as capital inflow or reserve use, and check whether it stems from a trade gap or from income and transfer flows.