Welcome to the World of Elasticity!

In the previous chapter, we learned that when the price of a product goes up, people usually buy less of it. But have you ever wondered how much less? If the price of salt doubles, will people stop buying it? Probably not. But if the price of a specific chocolate bar doubles, you might switch to a different brand instantly.

This "responsiveness" is what economists call Elasticity. By the end of these notes, you’ll be able to calculate and explain exactly how consumers react to changes in price, income, and the prices of other goods. Don't worry if the numbers look intimidating at first—we'll break them down step-by-step!

1. Price Elasticity of Demand (PED)

Price Elasticity of Demand (PED) measures how much the quantity demanded of a product changes when its price changes.

The Formula

To find the PED coefficient, use this formula:
\( \text{PED} = \frac{\% \Delta \text{ Quantity Demanded}}{\% \Delta \text{ Price}} \)
(Note: The Greek letter \( \Delta \) just means "change in".)

Understanding the Values

The answer you get from the formula tells you a story about the product:

  • Perfectly Inelastic (PED = 0): Quantity demanded doesn't change at all, no matter the price. Example: Life-saving medicine.
  • Inelastic (0 < PED < 1): Consumers aren't very sensitive to price. A big change in price leads to only a small change in demand. Example: Electricity or cigarettes.
  • Unitary Elasticity (PED = 1): The percentage change in demand is exactly equal to the percentage change in price.
  • Elastic (1 < PED < \infty): Consumers are very sensitive. A small price hike makes demand drop significantly. Example: Specific brands of soda.
  • Perfectly Elastic (PED = \infty): At a specific price, demand is infinite, but if the price rises even a tiny bit, demand drops to zero.

Quick Review: Think of a rubber band. An elastic demand "stretches" a lot when you pull the price. An inelastic demand is like a thick rope—it hardly moves!

Factors Affecting PED (The "SPLAT" Mnemonic)

Why are some things elastic and others isn't? Remember SPLAT:

  • S – Substitutes: More substitutes = More Elastic. (If Pepsi is expensive, buy Coke).
  • P – Percentage of Income: Cheap items (matches) are Inelastic; expensive items (cars) are Elastic.
  • L – Luxury vs. Necessity: Necessities are Inelastic; Luxuries are Elastic.
  • A – Addictiveness: Habit-forming goods (nicotine) are Inelastic.
  • T – Time: In the long run, demand becomes more Elastic as people find alternatives.

PED and Total Expenditure

Businesses use PED to decide whether to raise or lower prices to make more money (Total Revenue).
Total Expenditure = Price \times Quantity

  • If demand is Inelastic: Raising the price increases total expenditure.
  • If demand is Elastic: Lowering the price increases total expenditure.

Key Takeaway: If you sell something people need (inelastic), you can raise prices to earn more. If you sell something people can easily replace (elastic), you should be careful with price hikes!

2. Income Elasticity of Demand (YED)

Income Elasticity of Demand (YED) measures how demand changes when a consumer's income changes.

The Formula

\( \text{YED} = \frac{\% \Delta \text{ Quantity Demanded}}{\% \Delta \text{ Income}} \)

The Importance of the Sign (+ or -)

Unlike PED, the plus or minus sign in YED is very important!

  • Normal Goods (Positive YED): As you get richer, you buy more.
    • Necessities (0 < YED < 1): Demand grows slowly as income rises (e.g., milk).
    • Luxuries (YED > 1): Demand grows rapidly as income rises (e.g., designer handbags).
  • Inferior Goods (Negative YED): As you get richer, you buy less because you can afford better quality. Example: Instant noodles or public bus rides.
Did you know?

A "luxury" can become a "necessity" over time. 30 years ago, a mobile phone had a high YED (luxury). Today, for most people, it has a low YED (necessity).

Key Takeaway: If the economy is booming and incomes are rising, businesses selling luxury goods will see the biggest boost in sales!

3. Cross Elasticity of Demand (XED)

Cross Elasticity of Demand (XED) measures how the demand for one good (Good A) reacts to a change in the price of a different good (Good B).

The Formula

\( \text{XED} = \frac{\% \Delta \text{ Quantity Demanded of Good A}}{\% \Delta \text{ Price of Good B}} \)

The Relationship Between Goods

  • Substitutes (Positive XED): If the price of Coffee goes up, the demand for Tea goes up. People switch! (The stronger the substitute, the higher the number).
  • Complements (Negative XED): If the price of Printers goes up, the demand for Ink Cartridges goes down because they are used together.
  • Unrelated Goods (XED = 0): If the price of shoes changes, it has no effect on the demand for apples.

Common Mistake: Students often mix up the signs. Remember: Substitutes are Same direction (Positive). Complements go in Conflicting directions (Negative).

Key Takeaway: XED helps firms see who their real competitors are and how "jointly demanded" goods affect each other.

4. Variation in PED along a Straight-Line Demand Curve

Here is a tricky concept: Even if a demand curve is a straight line, the PED is not the same at every point.

  • At high prices (the top of the curve), demand is Elastic.
  • In the middle of the curve, demand is Unitary Elastic.
  • At low prices (the bottom of the curve), demand is Inelastic.

Why? Because a \$1 increase on a \$2 item is a 50% increase, but a \$1 increase on a \$100 item is only a 1% increase!

5. Why Does Elasticity Matter for Decision-Making?

Economists don't just calculate these for fun; they are vital for making choices:

For Firms:
  • Pricing: Should we have a sale? (Only if demand is elastic!)
  • Stocking: If we know incomes are falling (recession), should we stock more "inferior" budget-brand items? (Yes, YED tells us this).
  • Competition: If our rival cuts their price, how much will our sales drop? (XED tells us this).
For Governments:
  • Taxation: Governments want to tax goods with Inelastic demand (like alcohol or tobacco). Why? Because they know people will keep buying them even with the tax, which guarantees high tax revenue!

Final Encouragement: You've just covered one of the most important building blocks of Microeconomics! If you can master the difference between the size (the number) and the sign (+ or -), you are well on your way to an A.