# Elasticity

> Edexcel International GCSE Economics · 4EC1
> Source: https://www.owlsprep.com/study/edexcel-igcse-economics-s1-elasticity/

This guide covers all elasticity content for Edexcel IGCSE Economics (4EC1) section 1.1.4, including PED, PES and YED calculations, diagrams, value interpretations, influencing factors and real-world applications.

**Prerequisites:** [The market system: demand, supply and equilibrium](https://www.owlsprep.com/study/edexcel-igcse-economics-s1-demand-supply-equilibrium/)

## Learning objectives

- Define, calculate and interpret PED, PES and YED using given percentage changes
- Draw and label diagrams for elastic/inelastic demand and supply curves
- Explain key factors influencing the value of PED and PES
- Link PED to changes in total revenue for businesses
- Evaluate the significance of elasticity for firm pricing strategies and government tax/subsidy policy

## 1. Price Elasticity of Demand (PED)

**Price Elasticity of Demand (PED)** — Measure of how much quantity demanded of a good responds to a change in its own price

*Notation:* $PED = \frac{\% \Delta Q_d}{\% \Delta P}$

*Example:* If a 10% price rise leads to a 5% fall in quantity demanded, PED = -0.5

PED is always negative due to the inverse relationship between price and quantity demanded (law of demand). You can interpret values using the following scale:

- Perfectly inelastic: $PED = 0$ (quantity demanded does not change when price changes)
- Inelastic: $|PED| < 1$ (quantity changes less than proportionally to price)
- Unitary elastic: $|PED| = 1$ (quantity changes proportionally to price)
- Elastic: $|PED| > 1$ (quantity changes more than proportionally to price)
- Perfectly elastic: $PED = \infty$ (quantity demanded falls to zero if price rises even slightly)

> **tip**
>
> Steeper demand curves = more inelastic; flatter demand curves = more elastic. Perfectly inelastic demand is a vertical line, perfectly elastic is a horizontal line.

**Worked example:** The price of milk rises by 15% and quantity demanded falls by 3%. Calculate PED and interpret the value.

1. Step 1: Recall the PED formula

   $$PED = \frac{\% \Delta Q_d}{\% \Delta P}$$
2. Step 2: Substitute given values

   $$PED = \frac{-3\%}{+15\%} = -0.2$$
3. Step 3: Interpret: $|-0.2| < 1$, so demand for milk is price inelastic.

Four key factors influence PED: availability of close substitutes, degree of necessity of the good, percentage of consumer income spent on the good, and time period allowed for consumers to adjust to price changes.

> **Exam tip:** When drawing PED diagrams, always label axes (Price = y, Quantity = x) and explicitly label the curve as elastic or inelastic to secure all available marks.

*Calculator:* allowed

## 2. PED and Total Revenue

**Total Revenue (TR)** — Total income a firm generates from selling its goods, before subtracting costs

*Notation:* $TR = P \times Q$

The impact of a price change on total revenue depends entirely on PED:

- If demand is elastic: Price rise → TR falls; Price cut → TR rises
- If demand is inelastic: Price rise → TR rises; Price cut → TR falls
- If demand is unitary elastic: Price change has no impact on TR

**Worked example:** A bakery cuts the price of croissants from £2 to £1.80, and quantity sold rises from 50 per day to 75 per day. Calculate the change in TR and state if demand is elastic or inelastic.

1. Step 1: Calculate original TR

   $$TR_{original} = 2 \times 50 = £100$$
2. Step 2: Calculate new TR

   $$TR_{new} = 1.80 \times 75 = £135$$
3. Step 3: Change in TR = £135 - £100 = +£35. Price fell and TR rose, so demand is price elastic.

> **Exam tip:** For questions linking price changes to total revenue, always show both original and new TR calculations before stating the PED classification to get full marks.

*Calculator:* allowed

## 3. Price Elasticity of Supply (PES)

**Price Elasticity of Supply (PES)** — Measure of how much quantity supplied of a good responds to a change in its own price

*Notation:* $PES = \frac{\% \Delta Q_s}{\% \Delta P}$

PES is always positive due to the direct relationship between price and quantity supplied (law of supply). The value interpretation scale is identical to PED. Steeper supply curves = more inelastic; flatter curves = more elastic.

Key factors influencing PED: mobility of factors of production, availability of finished goods stocks, amount of spare production capacity, and time period allowed for producers to adjust output.

**Worked example:** The price of corn rises by 20% and quantity supplied increases by 4% in the short run. Calculate PES and explain why the value is low.

1. Step 1: Apply PES formula

   $$PES = \frac{+4\%}{+20\%} = 0.2$$
2. Step 2: 0.2 < 1, so supply of corn is price inelastic.
3. Step 3: Low PES because corn is a primary agricultural good that takes months to grow, so producers cannot increase output quickly in the short run.

> **Exam tip:** When comparing PES of primary and manufactured goods, always reference at least one influencing factor (e.g. production time lags for agricultural goods) to support your answer.

*Calculator:* allowed

## 4. Income Elasticity of Demand (YED)

**Income Elasticity of Demand (YED)** — Measure of how much quantity demanded of a good responds to a change in consumer income

*Notation:* $YED = \frac{\% \Delta Q_d}{\% \Delta Y}$

- Negative YED: Inferior good (demand falls as income rises, e.g. value supermarket goods)
- Positive YED < 1: Normal necessity good (demand rises slower than income, e.g. toothpaste)
- Positive YED > 1: Luxury good (demand rises faster than income, e.g. international holidays)

**Worked example:** Average household income rises by 10% and demand for budget takeaway meals falls by 5%. Calculate YED and classify the good.

1. Step 1: Apply YED formula

   $$YED = \frac{-5\%}{+10\%} = -0.5$$
2. Step 2: Negative YED means budget takeaway is an inferior good.

*Calculator:* allowed

## 5. Significance of Elasticity for Businesses and Governments

Elasticity data is used for key strategic decisions by both firms and governments:

- Businesses: Use PED to set pricing strategies (raise prices for inelastic goods to boost revenue, cut prices for elastic goods to increase sales volume). Use YED to plan production during economic booms and recessions.
- Governments: Use PED when imposing indirect taxes: taxes on inelastic goods (cigarettes, petrol) raise more revenue and cause smaller falls in quantity demanded. Use YED to forecast demand for public services during periods of income change.

**Worked example:** Explain why a government would choose to tax petrol rather than designer clothing to raise revenue.

1. Step 1: Petrol has price inelastic demand (few substitutes, necessary for transport for most households).
2. Step 2: A tax on petrol will lead to a small fall in quantity demanded, so the government will raise large tax revenue, while also incentivizing small reductions in carbon emissions.
3. Step 3: Designer clothing has elastic demand, so a tax would lead to a large fall in sales, raising very little revenue and harming clothing retailers.

> **Exam tip:** For evaluate questions, always include a counterpoint: e.g. while taxes on inelastic goods raise high revenue, they may disproportionately harm low-income households.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Forgetting the negative sign for PED and YED in calculations
  - Why it fails: The negative sign signals the inverse relationship between price and demand for PED, and inferior good status for YED. Omitting it leads to incorrect interpretation of values.
  - Correct: Always include the sign unless the question explicitly asks for the absolute value.
- **Wrong:** Mixing up PED and PES formulas or influencing factors
  - Why it fails: Both use % change in quantity over % change in price, but PED refers to quantity demanded while PES refers to quantity supplied, and their influencing factors are distinct.
  - Correct: Label $Q_d$ and $Q_s$ explicitly in formulas, and memorize factor lists separately for PED and PES.
- **Wrong:** Drawing inelastic curves as flat and elastic curves as steep
  - Why it fails: Steeper curves mean quantity changes very little for a given price change, which is the definition of inelastic, not elastic.
  - Correct: Use the mnemonic: Flat = Flexible = Elastic; Steep = Stuck = Inelastic.
- **Wrong:** Stating that price increases always raise total revenue
  - Why it fails: This is only true if demand is inelastic. If demand is elastic, a price increase will lead to a large fall in quantity sold, reducing total revenue.
  - Correct: Always link price changes to total revenue via the PED value before drawing a conclusion.
- **Wrong:** Confusing YED value interpretations for normal/inferior goods
  - Why it fails: Mixing up positive and negative YED values leads to incorrect classification of good types, losing marks on interpretation questions.
  - Correct: Use the mnemonic: N-I-P (Normal = Positive, Inferior = Negative), and YED > 1 = Luxury good.

## Cheatsheet

| Elasticity Type | Core Formula | Value Interpretations | Key Influencing Factors |
| --- | --- | --- | --- |
| PED | $\frac{\%\Delta Q_d}{\%\Delta P}$ | <1 Inelastic, =1 Unitary, >1 Elastic, 0 Perfectly Inelastic, ∞ Perfectly Elastic | Substitutes, Necessity, % of income spent, Time |
| PES | $\frac{\%\Delta Q_s}{\%\Delta P}$ | <1 Inelastic, =1 Unitary, >1 Elastic, 0 Perfectly Inelastic, ∞ Perfectly Elastic | Factor mobility, Stocks, Spare capacity, Time |
| YED | $\frac{\%\Delta Q_d}{\%\Delta Y}$ | <0 Inferior, 0<YED<1 Normal Necessity, >1 Luxury | N/A |

## What's next

Now that you have mastered elasticity concepts, you can apply this knowledge to analyse how demand and supply shifts affect market outcomes, including the impact of government interventions like indirect taxes and subsidies. Elasticity is a foundational concept for later topics in market failure and business strategy, so solidifying your understanding now will help you score high marks on extended response questions in Paper 1. Be sure to practice past paper calculation and diagram questions to build speed and accuracy for your exam.

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