Elasticity in Economics A Level: PED, PES, YED, and XED Explained

Elasticity is one of the most important and most frequently tested concepts across the entire CAIE Economics 9708 specification. It appears in multiple choice, data response, and essay questions at both AS Level (Papers 1 and 2) and A2 Level (Papers 3 and 4). It is also the concept where small errors of interpretation cost the most marks.

This guide covers all four types of elasticity, their formulas, their interpretation, their diagrams, and their application to real-world market scenarios.

What Is Elasticity?

Elasticity is a measure of how responsive one variable is to a change in another variable. In economics, we almost always measure how responsive quantity demanded or supplied is to a change in price, income, or the price of another good.

The general elasticity formula is always:

Elasticity = % change in quantity ÷ % change in the independent variable

1. Price Elasticity of Demand (PED)

Formula: PED = % change in Qd ÷ % change in Price

Interpretation of values:

• PED = 0: Perfectly inelastic (vertical demand curve) — quantity demanded does not change at all when price changes. Example: life-saving insulin. • 0 < PED < 1: Inelastic — quantity demanded changes by a smaller % than price. Example: petrol, cigarettes, electricity. • PED = 1: Unit elastic — quantity demanded changes by exactly the same % as price. • PED > 1: Elastic — quantity demanded changes by a larger % than price. Example: luxury goods, goods with many close substitutes. • PED = ∞: Perfectly elastic (horizontal demand curve) — any rise in price causes demand to fall to zero. Seen in perfectly competitive markets.

Note: PED is always negative (price and quantity demanded move in opposite directions) but economists typically refer to the absolute value.

What determines PED?

• Number and closeness of substitutes (more substitutes → more elastic) • Whether the good is a necessity or luxury (necessities → inelastic) • Proportion of income spent on the good (larger proportion → more elastic) • Time period (longer time → more elastic, as consumers adjust behaviour)

Revenue implications (critical for exam):

• If demand is inelastic (PED < 1): raising price increases total revenue; cutting price reduces revenue. • If demand is elastic (PED > 1): raising price reduces total revenue; cutting price increases revenue.

2. Price Elasticity of Supply (PES)

Formula: PES = % change in Qs ÷ % change in Price

Interpretation: PES is always positive (supply and price move in the same direction).

• PES < 1: Inelastic supply — producers cannot easily increase output in response to a price rise. Example: agricultural goods in the short run, houses. • PES > 1: Elastic supply — producers can rapidly increase output. Example: manufactured goods with spare capacity. • PES = 0: Perfectly inelastic — supply cannot change at all. Example: original Picasso paintings (fixed supply). • PES = ∞: Perfectly elastic — producers will supply any quantity at the existing price.

What determines PES?

• Time period (longer time → more elastic) • Availability of spare capacity (more capacity → more elastic) • Ease of storing the good (easier to store → more elastic) • Availability of factors of production (more available → more elastic)

3. Income Elasticity of Demand (YED)

Formula: YED = % change in Qd ÷ % change in Income

Interpretation:

• YED > 0: Normal good — demand rises as income rises. • YED > 1: Luxury good — demand rises faster than income (e.g., foreign holidays, designer goods). • 0 < YED < 1: Necessity — demand rises but slower than income (e.g., basic food). • YED < 0: Inferior good — demand falls as income rises (e.g., instant noodles, second-hand goods). As consumers get richer, they switch to superior substitutes.

Application: YED is crucial for businesses forecasting demand during economic booms and recessions, and for governments predicting tax revenues.

4. Cross Elasticity of Demand (XED)

Formula: XED = % change in Qd of Good A ÷ % change in Price of Good B

Interpretation:

• XED > 0: Substitutes — when the price of Good B rises, demand for Good A rises. Example: Pepsi and Coca-Cola, bus and train travel. • XED < 0: Complements — when the price of Good B rises, demand for Good A falls. Example: printers and ink cartridges, cars and petrol. • XED = 0: Goods are unrelated.

The higher the positive XED value, the closer the substitutes. This is highly relevant in competition policy — regulators use XED to determine whether two firms compete in the same market.

Elasticity in Tax Incidence Analysis

One of the most popular A2 Level applications of elasticity is the analysis of tax incidence — who bears the burden of a tax, producers or consumers?

• If demand is inelastic (consumers are not responsive to price changes): most of the tax burden falls on consumers. Producers can pass the tax on through higher prices without losing much demand. Example: taxes on cigarettes. • If supply is inelastic (producers cannot easily adjust output): most of the tax burden falls on producers. Example: a land value tax.

The diagram for this shows the tax as a vertical distance between the new supply curve (S + tax) and the original supply curve, and the incidence is split between the consumer price rise and the producer price fall.