Interpretation of the significance of the PED value: perfectly inelastic, inelastic, unitary, elastic, perfectly elastic

📊 Price Elasticity of Demand (PED) – The Allocation of Resources

What is PED?

PED measures how much the quantity demanded of a good changes when its price changes. Think of it like a rubber band: if you stretch it a little, it snaps back a lot (elastic), but if it’s very stiff, it barely moves (inelastic).

Formula: \$\displaystyle \epsilond = \frac{\% \Delta Qd}{\% \Delta P}\$ – the percentage change in quantity demanded divided by the percentage change in price.

How to Interpret PED Values

The sign of PED is always negative (law of demand). We focus on the absolute value:

  • Perfectly Inelastic (|ε| = 0) – Quantity demanded never changes no matter the price. Example: life‑saving medicine 💊.

  • Inelastic (0 < |ε| < 1) – Quantity changes, but less than the price change. Example: coffee ☕️ for regular drinkers.

  • Unitary Elastic (|ε| = 1) – Quantity changes proportionally to price. Example: a balanced trade‑off between price and quantity for a niche gadget 🔧.

  • Elastic (|ε| > 1) – Quantity changes more than the price change. Example: luxury watches 💎 when prices rise.

  • Perfectly Elastic (|ε| → ∞) – Consumers will buy any quantity at one price, but none at a higher price. Example: a perfectly competitive market for a commodity like wheat 🌾.

Why It Matters for Resource Allocation 📈

Governments and businesses use PED to decide how to set prices, taxes, or subsidies. If demand is elastic, a small price drop can lead to a big sales boost, freeing up resources for other uses. If demand is inelastic, price changes have little effect on sales volume, so revenue changes mainly with price.

Quick Reference Table

PED ValueInterpretationExample
|ε| = 0Perfectly InelasticLife‑saving medicine 💊
0 < |ε| < 1InelasticCoffee ☕️
|ε| = 1Unitary ElasticNiche gadget 🔧
|ε| > 1ElasticLuxury watch 💎
|ε| → ∞Perfectly ElasticWheat market 🌾

Real‑World Analogy: The “Elastic Band” 📏

Imagine a rubber band stretched between your fingers. If the band is thin (elastic), pulling it a little makes it stretch a lot – just like a product with elastic demand. If the band is thick (inelastic), pulling it a lot only gives a small stretch – like a necessity with inelastic demand.

Key Take‑aways for IGCSE Students

  1. Remember the formula: \$\displaystyle \epsilond = \frac{\% \Delta Qd}{\% \Delta P}\$.

  2. Use the absolute value of PED to classify demand.

  3. Think of real‑life examples to remember each category.

  4. Understand that PED helps explain how resources are allocated when prices change.

  5. Practice calculating PED with sample data to build confidence.

Practice Question 🚀

If the price of a video game drops from £50 to £40 (a 20% decrease) and the quantity demanded rises from 200 to 260 units (a 30% increase), what is the PED? Is the demand elastic, unitary, or inelastic?

Solution: \$\displaystyle \epsilon_d = \frac{30\%}{-20\%} = -1.5\$ → |ε| = 1.5 → Elastic demand. 📈