Which of these factors causes a change in the quantity demanded of a commodity?

Step 1: Analyze the question The question uses the specific phrase "change in the quantity demanded." In economics, this is different from a "change in demand," and examiners love testing this difference.

Step 2: Explain the concept

• Change in quantity demanded: This is caused ONLY by a change in the price of the commodity itself. It is shown by a movement along the same demand curve (an extension or contraction of demand).
• Change in demand: This is caused by other factors such as income, population, tastes and fashion, and prices of related goods (substitutes and complements). It is shown by a shift of the whole demand curve (an increase or decrease in demand).

Step 3: Go through the options

• A. Income: changes demand, so the curve shifts. Not the answer.
• B. Population: changes demand (more buyers in the market), so the curve shifts. Not the answer.
• C. Price of other commodities: affects demand through substitutes and complements, so the curve shifts. Not the answer.
• D. Price of the commodity concerned: if the price of, say, garri rises, people buy less garri. This is a movement along the demand curve, which is a change in quantity demanded. This is correct.
• E. Taste and fashion: changes demand, so the curve shifts. Not the answer.

Step 4: Answer The correct option is D. Price of the commodity concerned.

Step 5: Exam tip Remember this: "Price moves you along; everything else shifts the curve."

• Own price changes mean a movement along the curve (quantity demanded changes).
• Income, taste, population, and other prices mean a shift of the curve (demand changes).

Watch the exact wording in the question: "quantity demanded" points to own price, while "demand" points to the other factors.