Before you draw an arrow, decide which curve the event belongs to. If the event changes what buyers want or can afford, it moves demand. If it changes what sellers must pay or are able to produce, it moves supply.
This lesson belongs to price mechanism with more than one change. It assumes you can already tell a movement along demand from a shift.
What is the buyer-or-seller test?
Write the event in a sentence and finish it with “…so buyers…” or “…so sellers…”. The group that changes its decision owns the curve.
The test gives a direction as well. For demand, ask whether buyers want more or less at every price. For supply, ask whether sellers will offer more or less at every price.
Worked example: four events, one product
A fictional market sells adult bicycles in a city. The product is bicycles, so every event is judged for its effect on bicycles.
| Event | Who changes | Curve | Direction |
|---|---|---|---|
| The price of steel used in frames rises | Sellers pay more to produce | Supply | Left |
| The city opens new cycling lanes | Buyers want bicycles more | Demand | Right |
| The price of electric scooters falls | Buyers switch to a substitute | Demand | Left |
| A factory installs a faster welding machine | Sellers can produce more cheaply | Supply | Right |
Read the third row twice. The price that falls belongs to scooters, not to bicycles, so it cannot be a movement along the bicycle curve. It is a determinant of bicycle demand, so the bicycle demand curve shifts.
Now combine the first two rows. Supply shifts left and demand shifts right. Price rises in both effects, so price rises with certainty. Quantity is uncertain, and you can refer to what can and cannot be concluded when both curves shift.
The mistake that costs marks
A typical wrong answer to the steel row is “the price of steel rises, so demand for bicycles falls”. It feels natural because higher costs lead to higher bicycle prices, and higher prices lower quantity demanded.
The error is the route. The steel price does not change what buyers want. It changes what sellers must pay, so the supply curve shifts left. The higher bicycle price that follows is a result of that shift, and the lower quantity is a movement up the unchanged demand curve.
| Step | Wrong | Right |
|---|---|---|
| Event owner | Buyers | Sellers |
| Curve that shifts | Demand, left | Supply, left |
| Effect on price | Falls | Rises |
| Effect on quantity | Falls | Falls |
Notice that the wrong answer gets quantity right and price wrong. A diagram with the wrong curve can look convincing, which is why the test comes first.
Check yourself
For the bicycle market, name the curve and direction for each event. (a) A national cycling race raises interest in cycling. (b) The government lowers the price of the fuel that cars use. (c) A new producer enters the bicycle market.
Answer
(a) Buyers’ tastes change. Demand shifts right.
(b) Cheaper fuel makes cars, a substitute for cycling, cheaper to run. Some buyers switch away from bicycles. Demand shifts left. Note that the price that changed is fuel, not bicycles.
(c) More sellers are in the market, so at every price more bicycles are offered. Supply shifts right.
What to study next
Move on to checking an equilibrium calculation against the original diagram, then test the chapter with the cluster practice set. The supply-demand diagram reasoning explorer lets you try other events.
If you want a teacher to check which curve you choose and why, see online one-to-one Economics tuition.