Movement along vs shift in supply curve: what's the difference?
You learned that a price change doesn't shift the demand curve. The same rule governs supply — and students forget to carry it across.
You write: "the price rose, so the supply curve shifted right."
That sentence loses marks — the same way it does for demand. A change in the good's own price doesn't shift the supply curve. It moves you along it.
If you've already got the demand version straight, this is the same rule pointed at supply. Most students learn it once for demand and never carry it across.
Here's the fix, and it's one sentence: a change in the good's own price moves you along the supply curve. A change in a non-price determinant shifts it.
Same rule, second curve
A change in the good's own price causes a movement along the supply curve — a change in quantity supplied. The relationship between price and quantity hasn't changed; you've moved to a different point on the same curve.
The curve only shifts when a non-price determinant of supply changes. Then producers are willing and able to supply more or less at every price — and that's a new curve.
What actually shifts supply
The things that shift the supply curve are: the costs of factors of production; the prices of related goods (joint and competitive supply); indirect taxes and subsidies; future price expectations; technology; and the number of firms.
The good's own price is not on that list — same as it wasn't for demand.
The sentence the examiner wants
A change in the good's own price changes quantity supplied, not supply.
Where it costs you
It costs marks the same way the demand error does. Believe a price rise shifted supply and you'll draw a shifted curve when the answer is a movement up along it — wrong diagram in Paper 1, wrong written claim in Paper 2. And because supply questions often build into tax, subsidy, or equilibrium analysis, the error compounds through everything stacked on top.
Mia catches this in real time
Students don't lose marks from not knowing the topic. They lose marks because they learned the rule for demand and assumed supply was a separate thing to memorise.
When you tell Mia a price change shifted supply, she catches that you've stopped transferring the rule you already know — and makes you re-state it for supply before the gap shows up in an exam diagram.
Stop practising the wrong answer.
Mia spots the misconception, fixes the thinking, and makes you redraw it correctly.
Across the full IB Economics and Business Management curriculum. Free to start. No card needed.
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