CAPE Economics Unit 1 common mistakes
CAPE Economics Unit 1 · Understand the traps and how to avoid them.
When asked to distinguish a movement along a demand curve from a shift, the candidate labels a change in price as a shift (or a change in income as a movement), then draws the wrong arrow on the diagram and contradicts it in the text.
The candidate memorises 'price moves along, other things shift' as a slogan but never anchors it to the ceteris paribus clause: price is the variable on the axis, so a price change is endogenous and everything else is a shift parameter. Under time pressure the slogan is recalled without the reasoning, so the mapping is applied backwards.
What to do instead
Before writing, state the shift parameters explicitly (income, tastes, prices of related goods, expectations, number of buyers) and circle the axis variable. Then force the check: 'Did a number on an axis change, or did a curve move?' Practise 10 scenarios and for each write one sentence naming the determinant and one sentence naming the diagram effect, so the causal chain is rehearsed rather than the label.
The candidate calculates price elasticity of demand using the simple percentage formula and gets a different answer from the accepted one, or reports a value of 2.5 without stating it is elastic and without linking it to total revenue.
Many candidates apply (ΔQ/Q) ÷ (ΔP/P) with the original values only, ignoring the mid-point convention used for discrete changes, and treat the number as the end of the question rather than the start of an interpretation. The arithmetic is often done correctly but the economic meaning is never extracted.
What to do instead
Adopt one convention and show it: mid-point elasticity = (ΔQ ÷ average Q) ÷ (ΔP ÷ average P), with the negative sign dropped and the value labelled elastic, unit-elastic or inelastic. Always finish with the revenue implication (elastic price rise lowers revenue, inelastic price rise raises it) and a one-line example. Redo two past-style calculations using both conventions so the difference is visible.
On a supply-and-demand diagram the candidate shifts both curves in the same question, then asserts a definite new equilibrium price and quantity even though one of the two is indeterminate.
Candidates want a single clean answer and do not realise that when both curves move, the direction of change in one variable depends on the relative size of the shifts. The habit of 'draw it and read off the answer' fails precisely when the diagram has two competing effects.
What to do instead
Draw the two shifts separately first (shift demand only, note the effect; shift supply only, note the effect), then combine. State which variable is determinate and which is ambiguous, and say the outcome depends on the magnitude of the shifts. If the question supplies magnitudes, use them; if not, say so explicitly. This earns marks for analysis rather than losing them for a false certainty.
The candidate treats a price ceiling as automatically creating a shortage and a price floor as automatically creating a surplus, without checking whether the ceiling is set below or the floor above the equilibrium price.
Textbook diagrams usually show binding ceilings and floors, so the candidate generalises from the picture and skips the comparison with equilibrium. The conditional nature of the result is lost.
What to do instead
Write the test explicitly: a maximum price only binds if set below equilibrium; a minimum price only binds if set above. Then describe the consequence (excess demand and queuing or black markets; excess supply and surpluses or government purchase) and add one welfare point (deadweight loss, loss of consumer or producer surplus). Draw a non-binding case once so the contrast is fixed.
When explaining market failure the candidate lists 'externalities' or 'public goods' as the cause without specifying the divergence between private and social cost or benefit, and without showing the welfare loss on a diagram.
The terms are learned as vocabulary rather than as a mechanism. The candidate knows the label but cannot articulate that the market price reflects only private costs, so output is too high (negative externality) or too low (positive externality) relative to the social optimum.
What to do instead
Use a fixed three-step template for every externality answer: (1) name the spillover and who bears it, (2) state that MSC ≠ MPC or MSB ≠ MPB at the market quantity, (3) identify the deadweight-loss triangle between the market and social optimum. Practise on one negative production externality and one positive consumption externality until the template is automatic.
The candidate describes a public good as 'a good provided by the government' and therefore misclassifies education or health care as a public good, missing non-rivalry and non-excludability.
Everyday usage of 'public' (state-provided) collides with the technical definition (non-rival and non-excludable). Because the two meanings share a word, the candidate never notices the substitution.
What to do instead
Define the two characteristics separately and test goods against each: is one person's consumption reduced by another's, and can non-payers be excluded? Classify education as a merit good with positive externalities, not a public good, and say why (it is excludable and partly rival). Build a short table of six goods with rival/non-rival and excludable/non-excludable ticked, then state the implication for free-rider problems.
In market-structure questions the candidate asserts that a monopoly charges 'the highest possible price' or that a firm in perfect competition 'sets its own price', without tying the claim to the profit-maximising condition MC = MR and the demand curve the firm faces.
The structures are memorised as adjectives (many sellers, one seller, homogeneous product) rather than as constraints on the demand curve. The candidate never connects the shape of the firm's demand curve to its pricing power, so the conclusion floats free of the model.
What to do instead
For every structure, start from the demand curve the firm faces: perfectly elastic at market price for the competitive firm (price taker, P = MR), downward-sloping for the monopolist (price maker, MR below P). Then apply MC = MR and read price off the demand curve. Show that the monopolist's price is constrained by demand elasticity, not 'the highest possible'.
The candidate states that a firm in perfect competition earns abnormal profit in the long run, or that a monopolistically competitive firm earns zero profit forever, by mixing up the short-run and long-run adjustment mechanisms.
The short-run and long-run diagrams look similar, and the entry-and-exit mechanism that drives long-run normal profit is stated in one line of the textbook and easily forgotten. The candidate reproduces the short-run diagram and labels it long run.
What to do instead
Write the adjustment story in words before drawing: supernormal profit attracts entry, which shifts the market supply curve right and pushes price down until AR = AC and profit is normal; losses drive exit and the reverse. Then draw the long-run equilibrium with the AC curve tangent to the demand or price line. For monopoly, note that barriers prevent this adjustment, which is why long-run supernormal profit persists.
The candidate explains the demand for a factor of production by saying 'wages depend on how hard the job is' or 'on how much the worker needs', rather than deriving it from marginal revenue product.
Distribution theory is often studied last and thinly, so the candidate falls back on everyday intuitions about fairness and effort. The link from the firm's output market to the factor market (MRP = MPP × MR) is not rehearsed, so the technical answer is unavailable under exam pressure.
What to do instead
Derive factor demand from the firm's side: a profit-maximising firm hires until MRP equals the marginal factor cost, so the demand curve for labour is the MRP curve. Then explain shifts via the determinants of MPP (capital, technology, worker quality) and of MR (output price, market structure). Practise explaining why a nurse and a footballer earn different wages using MRP differences plus non-competing groups.
The candidate uses the terms 'economic rent' and 'transfer earnings' interchangeably, or says a worker's whole wage is economic rent, without identifying the margin at which supply becomes elastic.
Both terms describe parts of a factor payment, and the diagram (a vertical or upward-sloping supply curve) is the only place the distinction is visible. Without drawing the supply curve the candidate cannot see that transfer earnings are the opportunity cost and rent is the surplus above it.
What to do instead
Anchor the definitions to the supply curve: transfer earnings are the area under the supply curve up to the employed quantity (the payment needed to keep the factor in its current use); economic rent is the area above it. A perfectly inelastic supply gives all rent, a perfectly elastic supply gives none. Work through one diagram with an upward-sloping supply curve and shade and label both areas.