QCE Economics - Unit 1 - Market forces

Price elasticity of demand

Calculate and interpret PED, total revenue, determinants, midpoint choices and limitations.

Part of the free QCE Economics notes library for Unit 1: Market forces.

Updated 2026-08-13 - 6 min read

Price elasticity of demand measures proportional responsiveness:

$PED=\left|\frac{\%\Delta Q_d}{\%\Delta P}\right|$

Demand is elastic when $PED>1$, inelastic when $PED<1$ and unit elastic when $PED=1$. State the percentage base or midpoint method, interval and stable-demand assumption.

The total-revenue rule works only when movement is along a stable demand curve. With elastic demand, price and revenue move oppositely. With inelastic demand, they move together. Revenue is not profit: costs and capacity are absent.

Demand tends to be more elastic with close substitutes, a narrowly defined market, a larger budget share, luxury status and more adjustment time. Emergencies, contracts, habits and necessities can reduce short-run responsiveness.

Elasticity is not slope. Slope depends on measurement units, while elasticity is unit-free. A straight demand curve can have constant slope but different elasticity at different points because the percentage bases change.

To estimate PED from market data, match price and quantity periods and check promotions, income, competitor prices, availability and stockouts. Price may itself respond to demand, so two observations rarely prove causality.

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Classroom deep dive

The model in full

Price elasticity of demand measures responsiveness: $PED=\left|\frac{\%\Delta Q_d}{\%\Delta P}\right|$. Demand is elastic when $PED>1$, inelastic when $PED<1$ and unit elastic when $PED=1$. Percentage change is $\frac{\text{new}-\text{old}}{\text{old}}\times100$; where the direction or base choice could distort a comparison, the midpoint method can be declared. Elasticity is not the slope: it is unit-free and can vary along a linear demand curve. Total revenue $TR=P\times Q$ moves opposite price when demand is elastic and with price when demand is inelastic, ceteris paribus.

Concepts your explanation must connect

1. Demand tends to be more elastic with close substitutes, a larger budget share, luxury status, a narrowly defined market and more adjustment time; necessities and habit or contract constraints often reduce short-run responsiveness.
2. The total-revenue test infers the elasticity range from observed price and quantity movement, but it does not explain the cause and fails if demand itself shifts between observations.
3. Elasticity affects tax incidence, pricing, congestion policy and revenue forecasts; it does not by itself determine profit because costs and capacity also change.

Construct the reasoning, one link at a time

1. Link 1: Confirm that the comparison represents movement along a stable demand curve, identify old and new price and quantity and declare the percentage method.
2. Link 2: Calculate both percentage changes with signs and units, divide, then use the absolute value only for the elasticity magnitude.
3. Link 3: Classify the result and cross-check it using total revenue before and after: $TR_0=P_0Q_0$ and $TR_1=P_1Q_1$.
4. Link 4: Explain the determinant behind the responsiveness and apply it to a specified consumer, business or government decision without confusing revenue with welfare or profit.

After constructing the chain, inspect every arrow. An arrow should represent an incentive, accounting flow, behavioural response, curve movement or institutional constraint. If it represents only chronology, it has not yet explained the outcome. State the relevant market, decision-maker, time horizon and ceteris paribus condition where each matters.

Fully worked reasoning

Notice that the conclusion is narrower than the whole topic. It answers the supplied problem, preserves its units and assumptions, and does not convert a conditional model into a universal claim. In calculations, show the formula, substitution, working and unit. In graphs, label axes, original and new curves, equilibrium points and the direction of change.

Economic inquiry workshop

QCAA's inquiry process moves through forming, selecting, analysing and evaluating. Use that sequence rather than collecting data first and inventing a question later.

Form. How responsive is demand for a selected product across time or customer groups?

Select. Use a genuine price variation, define the market narrowly and collect matched quantity, price, promotion, income, competitor and availability data for comparable periods.

Analyse. Calculate PED with a declared method, total-revenue change and uncertainty across segments; inspect whether demand determinants changed simultaneously.

Evaluate. Observed price may respond to demand rather than cause it, while stockouts censor quantity. Natural experiments or controlled trials improve identification but require ethical and commercial safeguards.

For every dataset, record publisher, exact series or table, units, observation period, release date, adjustment basis and revision status. Current evidence means the newest appropriate observation available—not merely a recently updated webpage. Triangulate the model with direct determinant evidence and at least one plausible competing explanation.

Misconception clinic

Repair: PED is a proportional magnitude conventionally reported positive; slope depends on units, elasticity varies with the price–quantity point, and revenue omits costs and distributional effects.

A useful correction names the first broken definition or causal link, rebuilds from that point, and explains why the revised conclusion follows. Adding terminology to the original claim without changing its logic is not a repair.

Unfamiliar transfer

For a tax, fare rise or discount in a new market, calculate rather than label elasticity, check the stable-demand assumption and separate revenue, profit, access and welfare conclusions.

Use this response frame:

1. Define the unfamiliar context and the relevant economic variable.
2. Reconstruct the model rather than copying the worked conclusion.
3. Calculate or graph the change with labels and units.
4. Trace the mechanism and identify a countervailing channel.
5. Evaluate against a named criterion for specified stakeholders and time horizons.
6. State which fresh evidence would resolve the remaining uncertainty.

Final verification: Show both percentage calculations, formula, magnitude classification and total-revenue cross-check; name the interval, method and at least one determinant or confounder.

Syllabus mapping

This lesson explicitly addresses the following mapped QCAA statements:

• Explain price elasticity of demand and the difference between elastic, inelastic and unitary elasticity, construct appropriate diagrams and apply to various situations
• Explain the factors affecting elasticity of demand, including necessities and luxuries, the existence of substitutes, the proportion of income spent on the good, and the length of time following a price change.
• Calculate the percentage change in prices from one period to the next, using the calculation for change
• Calculate the price elasticity of demand using the total revenue and/or point method, explain the numerical values, and relate to real economic scenarios.
• Analyse the significance of price elasticity of demand to consumers, business and government.

Sources

• QCAA, Economics 2025 v1.4 syllabus (March 2026)
• QCAA, Economics senior subject page
• QCAA, Economics 2025 v1.4 syllabus amendment report

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