Price Elasticity of Demand
Price elasticity of demand measures how sensitive consumers are to a change in price. It tells you the percentage change in quantity demanded resulting from a 1% change in price.
Key Takeaways
- Formula: PED = % Change in Quantity Demanded ÷ % Change in Price. The result is always negative (but we use the absolute value).
- If |PED| > 1, demand is elastic — consumers are very responsive to price changes. A price increase causes a large drop in quantity demanded.
- If |PED| < 1, demand is inelastic — consumers are not very responsive. Price changes have a small effect on quantity demanded.
- If |PED| = 1, demand is unit elastic — the percentage change in quantity exactly equals the percentage change in price.
Understanding Elasticity (and Why It Matters)
Price elasticity of demand (PED) answers a simple but powerful question: if I raise the price of this product by 10%, how much will sales drop? The answer depends on how many substitutes exist, whether the good is a necessity, and what share of the consumer's budget it represents.
The formula is straightforward: PED equals the percentage change in quantity demanded divided by the percentage change in price. Since price and quantity move in opposite directions along a demand curve, the result is always negative — but economists typically report the absolute value.
When |PED| is greater than 1, we say demand is elastic. This means consumers are very price-sensitive — a small price increase causes a proportionally larger decrease in quantity demanded. Goods with lots of substitutes tend to have elastic demand. Think: Coca-Cola (because you can switch to Pepsi).
When |PED| is less than 1, demand is inelastic. Consumers don't change their buying behavior much when prices change. Necessities like insulin, gasoline, or electricity tend to be inelastic — you need them regardless of price.
The key insight for exams is the relationship between elasticity and total revenue. If demand is elastic, raising the price actually decreases total revenue (because quantity drops so much). If demand is inelastic, raising the price increases total revenue. This is why firms care deeply about elasticity — it determines their pricing strategy.
There's also the midpoint method, which gives a more accurate elasticity calculation by averaging the two prices and quantities. The AP exam expects you to know this formula: PED = [(Q₂ - Q₁) / ((Q₂ + Q₁)/2)] ÷ [(P₂ - P₁) / ((P₂ + P₁)/2)].
Elasticity on the AP Micro Exam
Elasticity is tested heavily in Unit 2 of AP Microeconomics. Expect both calculation questions (using the midpoint method) and conceptual questions about the relationship between elasticity and total revenue.
The total revenue test is the most commonly tested concept: if price increases and total revenue increases, demand is inelastic. If price increases and total revenue decreases, demand is elastic. You should be able to apply this in both directions.
For FRQs, you might be asked to calculate elasticity using the midpoint method, then explain what the result means for the firm's pricing strategy. Always show your work — partial credit is available for correct setup even if the final answer is wrong.
The AP exam also tests cross-price elasticity (substitutes have positive cross-price elasticity, complements have negative) and income elasticity (normal goods are positive, inferior goods are negative). Know the signs and what they mean.
Common Mistakes Students Make
- Forgetting to use the midpoint method. The AP exam expects the midpoint formula, not the simple percentage change formula. Using the wrong method gives a different answer.
- Confusing elastic and inelastic with the total revenue test. Remember: elastic demand means a price increase decreases revenue. Draw it out if you're unsure.
- Thinking elasticity is the same as slope. Elasticity changes along a straight-line demand curve — it's not constant. A steep curve isn't necessarily inelastic.
Related Topics
Frequently Asked Questions
The midpoint method calculates elasticity using the average of the two prices and quantities as the base, rather than the starting point. The formula is: PED = [(Q₂ - Q₁) / ((Q₂ + Q₁)/2)] ÷ [(P₂ - P₁) / ((P₂ + P₁)/2)]. This gives the same result regardless of which direction you calculate (price increase vs decrease).
Demand tends to be more elastic when: (1) more substitutes are available, (2) the good takes up a larger share of the consumer's budget, (3) more time passes (consumers can find alternatives), and (4) the good is a luxury rather than a necessity.