A change in the price of the good causes a movement along the supply curve. A change in anything else that affects sellers shifts the curve. This distinction is tested in most Economics papers, in short answers, data responses and written explanations.
It is the first skill in supply and equilibrium, and it builds on the same idea you met in demand relationships.
What is the difference, step by step?
Supply is the relationship between price and the quantity sellers are willing to offer. The curve or schedule shows that relationship, so a different price just means reading a different point on it.
- Identify what changed. Is it the price of this good, or is it something behind the curve?
- If the price changed, read along the same schedule. Quantity supplied rises when the price rises. The curve itself does not move.
- If something else changed, such as the cost of materials, wages, technology or the number of sellers, the quantity offered at every price changes. The whole curve shifts.
- Name the result correctly. Use “a rise in quantity supplied” for movements and “an increase in supply” for shifts.
Worked example
Port Merah is a fictional town where sellers offer reusable lunch boxes.
| Price (RM) | Quantity supplied (before) | Quantity supplied (after plastic cost rises) |
|---|---|---|
| 10 | 20 | 6 |
| 15 | 40 | 26 |
| 20 | 60 | 46 |
| 25 | 80 | 66 |
| 30 | 100 | 86 |
Situation A. The price rises from RM20 to RM25 and nothing else changes. Quantity supplied rises from 60 to 80. This is a movement along the original supply schedule. In percentage terms, price rose 25% and quantity supplied rose 33.3%.
Situation B. The price of plastic rises, so each box costs more to make. At RM20, sellers now offer 46 instead of 60. At RM25 they offer 66 instead of 80. The gap is 14 at every price, which is the sign of a shift. The curve has moved left, and this is a decrease in supply.
Notice the test: in Situation A, the same price gives the same quantity. In Situation B, the same price gives a different quantity.
What mistake do students make?
Mistaken answer: “Sellers raised the price to RM25, so supply increased from 60 to 80.”
The sentence confuses cause and result. The price rose, which is a change in the price of the good itself. Sellers moved along their existing curve, and the correct wording is “quantity supplied rose from 60 to 80”.
The correction is to ask, before writing anything, “Did the thing that changed come from the price axis, or from somewhere behind the curve?” If it came from the axis, say quantity supplied.
Check yourself
1. In Port Merah, a new factory opens and sellers offer 10 more boxes at every price. Is this a movement or a shift? Which way?
Show answer
A shift, to the right, because the quantity offered changes at every price. This is an increase in supply.
2. Using the first schedule above, the price falls from RM25 to RM15. What happens to quantity supplied, and does the curve move?
Show answer
Quantity supplied falls from 80 to 40. The curve does not move. It is a movement down along the same curve, caused by the price change.
3. A student writes: “Because of a government grant for new machines, quantity supplied increased.” Correct the wording and give the reason.
Show answer
A grant for machines lowers the cost of production, which is a factor behind the curve. The wording should be “supply increased”, shown as a shift right, because more is offered at every price.
Where this leads next
Once you can name each change correctly, move on to finding an equilibrium from supplied schedules and test yourself with the supply and equilibrium practice set. The percentage-base explorer helps with the percentage changes used above.
Some students follow the diagram but still lose marks on the wording. That is the kind of pattern our teachers look for in online one-to-one Economics tuition.