Change in Quantity Supplied Vs. Change in Supply

1. The Key Question

When the quantity producers supply changes, the first question to ask is: What caused the change?

If the price of the commodity itself changes, there is a change in quantity supplied. If a factor other than price changes, there is a change in supply.

2. Change in Quantity Supplied

A change in quantity supplied is a movement from one point to another on the same supply curve caused by a change in the commodity’s price, while other factors remain constant.

• Price increases → quantity supplied increases.
• Price decreases → quantity supplied decreases.
• The supply curve itself does not shift.

Gari Example

If the price of gari falls from GH₵3 to GH₵2 and the quantity supplied falls from 10 units to 4 units, the change is a movement down the same supply curve.

3. Change in Supply

A change in supply occurs when the entire supply curve shifts because of a change in a factor other than the commodity’s own price.

Factors identified include:

• Production costs
• Technology
• Number of suppliers
• Government policies and regulations
• Expectations of future prices
• Prices of related goods
• Natural conditions

A favourable change in these factors can increase supply, causing a rightward shift. An unfavourable change can decrease supply, causing a leftward shift.

4. The Difference at a Glance

Change in Quantity Supplied	Change in Supply
Caused by a change in the commodity’s price.	Caused by a change in a non-price factor.
Movement along the same supply curve.	Shift of the entire supply curve.
Price increases → quantity supplied increases.	Favourable non-price change → supply increases.
Price decreases → quantity supplied decreases.	Unfavourable non-price change → supply decreases.

5. Work Through Two Situations

Situation A: Price Changes

The price of gari falls from GH₵3 to GH₵2. Quantity supplied falls from 10 units to 4 units.

Conclusion: This is a change in quantity supplied because the commodity’s own price changed. The movement is along the same supply curve.

Situation B: Technology Changes

A new technology is introduced in gari production, making production more favourable. Supply increases at the same price level.

Conclusion: This is a change in supply. The entire supply curve shifts to the right.

6. A Simple Decision Rule

7. Practise

1. The price of a commodity rises.
   Answer: Change in quantity supplied; movement up the same supply curve.
2. Production costs fall.
   Answer: Change in supply; supply increases and the curve shifts right.
3. The price of a commodity falls.
   Answer: Change in quantity supplied; movement down the same supply curve.
4. New technology improves production.
   Answer: Change in supply; the supply curve shifts right.
5. Natural conditions become unfavourable.
   Answer: Change in supply; supply decreases and the curve shifts left.

8. Final Check

Change in quantity supplied = price changes → movement along the same curve.

Change in supply = non-price factors change → the whole curve shifts.

9. Summary

The difference between the two concepts depends on the cause of the change. A change in the commodity’s own price changes quantity supplied and produces movement along the existing supply curve. A change in a non-price factor changes supply and shifts the entire curve to the right or left.