Main influences on whether demand is elastic or inelastic

What is Price Elasticity of Demand?

Price elasticity of demand measures how much the quantity demanded of a good changes when its price changes. It is calculated as:

\$\displaystyle \epsilond = \frac{\% \Delta Qd}{\% \Delta P}\$

Where:

• \$\% \Delta Q_d\$ = percentage change in quantity demanded
• \$\% \Delta P\$ = percentage change in price

Interpretation:

• Elastic demand: \$|\epsilon_d| > 1\$ – quantity changes a lot when price changes.
• Inelastic demand: \$|\epsilon_d| < 1\$ – quantity changes little when price changes.
• Unitary elasticity: \$|\epsilon_d| = 1\$ – quantity changes proportionally to price.

Key Influences on Elasticity

1. Availability of Substitutes – More substitutes → more elastic.

   Analogy: Choosing between different brands of cereal.

2. Proportion of Income Spent – Big‑ticket items (e.g., cars) → elastic; small items (e.g., salt) → inelastic.
3. Time Period – Short run: less elastic; long run: more elastic as consumers find alternatives.
4. Necessity vs Luxury – Necessities (e.g., medicine) → inelastic; luxuries (e.g., designer shoes) → elastic.
5. Definition of the Market – Narrow vs broad market definitions affect elasticity.

Examples & Analogies

Imagine a rubber band (elastic) and a piece of clay (inelastic). If you pull the rubber band (increase price), it stretches a lot (quantity demanded drops sharply). The clay resists stretching (quantity demanded barely changes).

📚 Real‑world example: If the price of coffee rises by 10%, the quantity demanded might drop by 15% (elastic). If the price of water rises by 10%, the quantity demanded drops by only 2% (inelastic).

Exam Tips

• Always calculate the absolute value of elasticity when classifying.
• Use the elasticity formula and plug in the correct percentage changes.
• Explain why a good is elastic or inelastic using the five influences.
• Remember that price increases lead to a negative elasticity (due to the law of demand).
• Practice with multiple-choice and short answer questions on elasticity scenarios.