To trace a policy mechanism, start with the model you are using, find the curve the instrument moves, and follow the shift to the new price and quantity. Every step must have a reason.
This lesson belongs to government and the economy. It uses the instrument labels from distinguishing policy objectives and instruments and the equilibrium skills from drawing market equilibrium.
What are the five steps?
- State the model and what is held constant.
- Name the instrument.
- Say which curve it moves and in which direction.
- Read off the new price and quantity.
- State a limit.
Worked example: a rice subsidy in an invented country
Zenia pays rice producers a subsidy of RM0.50 per kilogram. Before the subsidy, the market price is RM4.00 per kilogram and 600 tonnes are traded each week.
Step 1, the model. The market for rice, with demand and supply curves. Assume demand, other costs and the number of sellers stay constant.
Step 2, the instrument. A subsidy to producers.
Step 3, the curve. The subsidy lowers producers’ cost per kilogram, so supply shifts right. Demand does not move.
Step 4, the result. At the old price there is now more rice offered than wanted, so the price falls. In this illustration, the new equilibrium is RM3.60 and 680 tonnes. Price falls RM0.40, which is 10%, and quantity rises 80 tonnes, which is 13.3%, from 600 to 680.
Step 5, the limit. The size of the changes depends on how strongly buyers and sellers respond. If producers cannot grow more rice quickly, the quantity rises by less.
The figures in step 4 are read from an illustrative diagram, not data. The word “illustration” tells the reader they are an example.
The mistake to avoid
The common slip is to jump from instrument to result. Compare the two versions.
| Jump to the result | Full chain | |
|---|---|---|
| Instrument | Subsidy | Subsidy to producers |
| Curve | (missing) | Supply shifts right, demand unchanged |
| Result | “Farmers are better off” | Price falls and quantity rises |
| Limit | (missing) | Depends on how far output can grow |
The jump also slips in an unsupported claim, “farmers are better off”, which the model has not tested. Stick to what the curves show. A related slip is shifting demand instead of supply, so always ask who is directly affected first.
Variation: a tax on producers
If the instrument is a per-kilogram tax on producers, step 3 changes: the cost per kilogram rises, so supply shifts left. The price rises and the quantity falls. The five steps stay the same, which is the point of learning them as a routine.
You can test the direction of a shift in the supply-demand diagram reasoning explorer before writing your answer.
Check yourself
Zenia puts a tax of RM1 per kilogram on imported cheese sold by local shops. Trace the mechanism in four short sentences, and state one assumption.
Answer
Assumption: the market for cheese, with demand and other costs constant.
The tax raises the shops’ cost per kilogram, so supply shifts left. At the old price the quantity demanded exceeds the quantity supplied, so the price rises. The quantity traded falls.
Check: the answer names the curve, the direction, and the two changes in the equilibrium, and does not claim anything about who is better off.
What to study next
Go to separating model predictions from documented outcomes, which tests what a traced prediction can and cannot show. Then use the chapter practice.
If you want a teacher to watch your five steps live, see online one-to-one Economics tuition.