Topics: Microeconomics | Demand & Supply | Market Mechanism
Level: JC A-Level Economics / IB Economics
“70% of Economics students will make this mistake at least once.”
Are you one of them? Using the wrong term—Quantity Demanded instead of Demand—can cost you crucial marks in your essays because it implies a completely different economic mechanism.
In this lesson, we clarify the difference once and for all so you can join the top 30% of students who get it right.
Watch the video explanation with the accompanying notes below:
1. Change in Quantity Demanded (Qd)
Definition: Refers to a specific point on the Demand curve.
The Cause: A change in the Price (P) of the good itself.
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The Mechanism:
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If Price Rises (P1 -> P2): Quantity Demanded Falls (Q1 -> Q2).
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If Price Falls: Quantity Demanded Rises.
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The Diagram: This is represented by a Movement ALONG the curve. You are moving from one point to another on the same line.
Key Rule: If the price of the good changes, NEVER say “Demand changed.” Always say “Quantity Demanded changed.”
2. Change in Demand (D)
Definition: Refers to the entire relationship between price and quantity.
The Cause: A change in Non-Price Factors (e.g., Taste, Income, Population).
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The Mechanism:
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If a factor changes (e.g., Tastes shift in favor): Consumers are now willing to buy more at every single price level.
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The Result: The entire curve moves.
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The Diagram: This is represented by a Shift OF the curve.
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Increase in Demand: Shift Right (D1 -> D2).
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Decrease in Demand: Shift Left (D2 -> D1).
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Acronym Trick: In our classes, we use the acronym PET PIGS to remember the non-price factors (Population, Expectations, Tastes, Price of Related Goods, Income, Govt Policies, Seasonal factors).
3. Summary Table: The Difference
| Feature | Change in Quantity Demanded | Change in Demand |
| Visual | Movement Along the curve | Shift Of the entire curve |
| Cause | Change in Price (of the good itself) | Change in Non-Price Factors |
| What changes? | Moving from Point A to Point B | The whole line (D1 to D2) |
| Example | Price of iPhone drops -> Qd rises. | Incomes rise -> Demand for iPhones increases. |
4. The “Trick Question”: Price of Related Goods
The video poses a tricky question at the end:
“If there is a change in the price of a related good, does it change Quantity Demanded or Demand?”
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The Answer: It changes Demand.
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Why? The “Price” that causes a movement along the curve must be the price of the good itself. The price of a related good (Substitute or Complement) is considered an external factor.
Example:
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If the Price of Pepsi (Substitute) rises…
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People switch away from Pepsi and buy more Coke.
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This causes an Increase in Demand for Coke (Shift Right of the Coke Demand Curve).
Stop Losing “Silly Marks” in Economics
Mixing up “Movement” and “Shift” is the #1 reason students lose marks in Case Study Questions (CSQ). Don’t let definitions trip you up.
Join our JC Economics Tuition A-Level or IB Economics Tuition classes to master the technical rigor required for distinctions.
(For a full list of Non-Price Determinants, check our free Demand & Supply Notes. For other topics, browse our full Economics Notes Library.)