Welcome to the World of Elasticity!

In our previous look at demand, we learned a simple rule: when prices go up, people buy less. But have you ever wondered how much less? If a cinema raises ticket prices by $1, will the theater be empty, or will people barely notice? This is where elasticity comes in.

Think of elasticity like a rubber band. If a product’s demand is "elastic," it stretches a lot when you pull on the price. If it’s "inelastic," it’s like a thick, tough piece of string that hardly moves at all. In this chapter, we’ll explore how sensitive consumers are to changes in price, income, and the prices of other goods.

1. Price Elasticity of Demand (PED)

Price Elasticity of Demand (PED) measures how much the quantity demanded of a good changes when its price changes. It tells us if consumers are "price-sensitive."

The Formula

To calculate PED, we use this formula:

\(\text{PED} = \frac{\% \text{ change in Quantity Demanded}}{\% \text{ change in Price}}\)

How to Interpret the Numbers

Don't worry if the math seems scary! You just need to look at the result:

  • Elastic (PED > 1): Consumers are very sensitive. A small price change leads to a big change in demand (e.g., luxury chocolates).
  • Inelastic (PED < 1): Consumers aren't very sensitive. Even a big price change doesn't change demand much (e.g., electricity or addictive goods).
  • Unitary Elastic (PED = 1): The change in demand is exactly proportional to the change in price.

The "Negative" Sign Rule

Because price and quantity move in opposite directions (Law of Demand), PED is almost always a negative number. However, economists usually ignore the minus sign and just look at the "absolute value." If you get -2.5, we just say the PED is 2.5 (Elastic).

Factors that Influence PED

Why are some things more elastic than others? Remember the acronym S.P.L.A.T.:

  • S – Substitutes: If there are many other brands to choose from, demand is Elastic.
  • P – Percentage of Income: If it's a very cheap item (like a box of matches), demand is Inelastic. If it's expensive (like a car), it's Elastic.
  • L – Luxury or Necessity: Necessities (medicine) are Inelastic; luxuries (designer bags) are Elastic.
  • A – Addictiveness: Habit-forming goods (cigarettes) are Inelastic.
  • T – Time: In the long run, people find alternatives, making demand more Elastic.

PED and Total Revenue

This is a favorite exam topic! Firms use PED to decide whether to raise or lower prices to make more money (Total Revenue).

  • If demand is Elastic: Lowering the price increases total revenue. (You sell many more units).
  • If demand is Inelastic: Raising the price increases total revenue. (People keep buying even though it's more expensive).

Quick Review Box:
Elastic = Sensitive (stretchy rubber band).
Inelastic = Unresponsive (tough string).
To increase revenue: Lower price if elastic; Raise price if inelastic.

2. Income Elasticity of Demand (YED)

Income Elasticity of Demand (YED) measures how demand changes when consumer incomes rise or fall. In this formula, "Y" stands for Income.

The Formula

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

Normal vs. Inferior Goods

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

  • Normal Goods (Positive YED): As you earn more, you buy more of these.
    • Necessities (YED between 0 and 1): You buy a bit more, but not a huge amount (e.g., milk, fruit).
    • Luxuries (YED > 1): When your income jumps, you spend a lot more on these (e.g., international holidays, fine dining).
  • Inferior Goods (Negative YED): As you earn more, you buy less of these because you can now afford something better (e.g., instant noodles, public bus rides).

Did you know?
During an economic recession (when incomes fall), supermarket "budget brands" often see their sales go up! This is because they are inferior goods.

Key Takeaway:
Positive YED = Normal Good.
Negative YED = Inferior Good.

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). This helps businesses understand their competitors and partners.

The Formula

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

Substitutes and Complements

Again, the sign tells you the relationship between the two goods:

  • Substitutes (Positive XED): These are rival goods. If the price of Pepsi goes up, the demand for Coca-Cola goes up. (Consumers switch). The stronger the substitute, the higher the positive number.
  • Complements (Negative XED): These are goods used together. If the price of printers goes up, the demand for printer ink goes down. (They are "joined at the hip").
  • Unrelated Goods (XED = 0): A change in the price of cheese has no effect on the demand for iPhone apps.

Common Mistake to Avoid:
Students often mix up the signs. Remember:
Substitutes = Same direction (Price of B up, Demand for A up) = Positive.
Complements = Contrary direction (Price of B up, Demand for A down) = Negative.

Summary Checklist

Before you move on, make sure you can:

  • Calculate all three elasticities using the percentage change formulas.
  • Explain why a firm with inelastic demand might decide to raise its prices.
  • Identify Normal vs. Inferior goods based on YED.
  • Distinguish between Substitutes and Complements using XED.
  • List the S.P.L.A.T. factors that make demand more or less elastic.

Don't worry if these formulas feel a bit "maths-heavy" at first. Practice a few calculations with real numbers, and you'll soon see that elasticity is just a way to put a number on common-sense human behavior!