To compare two demand curves fairly, make sure both use identical axes and scales, read both at the same price, and describe the difference with numbers. Then decide whether it is a shift or a difference in steepness.
This lesson extends movement versus shift and sits in demand relationships. The demand, supply and shift explorer draws two curves on a shared scale, which makes the idea easy to see.
What do you check, step by step?
- Axes and units: same price units (RM) and the same quantity units (cups per day).
- Scale marks: equal steps on each axis in both graphs.
- Read both curves at the same price and write the two quantities.
- Find the gap at two or three prices.
- Classify: equal gap at every price means a parallel shift. A changing gap means the curves differ in steepness.
Worked example
The fictional Seri stall’s lime tea has demand A: Qd = 160 − 20P. In the nearby fictional town of Pulau Ketam Baru, the same drink has demand B: Qd = 100 − 20P. A third market has demand C: Qd = 160 − 40P.
| Price (RM) | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| A: 160 − 20P | 140 | 120 | 100 | 80 |
| B: 100 − 20P | 80 | 60 | 40 | 20 |
| C: 160 − 40P | 120 | 80 | 40 | 0 |
A against B. The gap is 140 − 80 = 60 at RM1, 120 − 60 = 60 at RM2, 100 − 40 = 60 at RM3, and 80 − 20 = 60 at RM4. The gap is constant, so B is a parallel shift left of A by 60 cups. Causes could be fewer buyers or lower income.
A against C. The gap is 20 at RM1, 40 at RM2, 60 at RM3, 80 at RM4. The gap grows with price, so the curves are not parallel. In market C quantity falls by 40 for each RM1, against 20 in A, so buyers in C respond more strongly to price. On a graph with the same axes, C is the flatter line.
Only because the scales are identical can you trust that picture. If C were drawn with a squeezed price axis, it might look steeper even though the table says otherwise.
The mistake to watch for
Mistaken answer: “Curve C is steeper because it looks steeper on the page.”
This judges by appearance, and the two graphs may use different scales. Correction: “Read both at RM3: A gives 100 and C gives 40.
The gap widens as price rises, so C responds more to price. On equal scales C is flatter.” Lean on the numbers, then describe the shape.
Check yourself
Use the table above.
1. Compare A and B at RM2 in one sentence.
Show answer
At RM2 demand A is 120 cups and B is 60 cups, so B is 60 cups lower, which is a leftward shift of the whole curve.
2. At what price does each of A and C reach zero quantity?
Show answer
A: 160 − 20P = 0 gives P = RM8. C: 160 − 40P = 0 gives P = RM4.
3. Is the gap between A and C the same at every price? What does it mean?
Show answer
No. The gap is 20, 40, 60, 80 at RM1 to RM4. It is not constant, so the curves differ in steepness, not just position: buyers in C react more to price.
Where this leads next
Continue to separate individual and market demand, then test everything on the mixed practice set. To compare responses between markets precisely, the topic of elasticity comes later.
If your comparisons stay vague, our online one-to-one Economics tuition can turn them into precise, evidence-led sentences.