A chain from evidence to a model starts with a figure in the data and ends with a conclusion about a market, using each step as a reason for the next. This skill appears when a data question asks you to “explain” or “analyse” a change, and it builds on supply and equilibrium and demand relationships. It is part of economics data responses.
What are the links in the chain?
Think of four links, each in its own sentence.
- Evidence: what the data shows, with figures.
- Interpretation: what that pattern suggests about demand or supply.
- Model step: which curve shifts, in which direction, and why.
- Result: the new equilibrium price and quantity compared with the old.
Missing any one link leaves a gap the examiner has to fill, and the answer cannot earn the credit that the gap would have taken.
How do you work from data to model?
- Write down the changes in price and quantity, with direction.
- Use the pairing. Price up and quantity up points to a rise in demand. Price down and quantity up points to a rise in supply. Price up and quantity down points to a fall in supply. Price down and quantity down points to a fall in demand.
- Name a cause that fits the context given in the question.
- Say which curve shifts, then describe the movement along the other curve.
- State the result with numbers where you have them.
Worked example
In Tamarind Bay, a fictional seaside town, a heatwave lasted a month. Weekly figures for coconut water were as follows.
| Before | During | |
|---|---|---|
| Price per bottle | RM4 | RM5 |
| Bottles sold per week | 800 | 1,000 |
| Weekly revenue | RM3,200 | RM5,000 |
Evidence. Price rose by RM1, which is 1 ÷ 4 = 25%, and quantity sold rose by 200 bottles, which is 200 ÷ 800 = 25%. Revenue rose from 4 × 800 = RM3,200 to 5 × 1,000 = RM5,000.
Interpretation. Price and quantity moved up together. A fall in supply would raise price but lower quantity, so the data points to a rise in demand.
Model step. The heatwave made more people want cold drinks at every price, so the demand curve shifts to the right. Demand is above supply at the old price of RM4.
Result. The shortage at RM4 pushes price up. As price rises, suppliers offer more, a movement along the supply curve, until a new equilibrium is reached at RM5 and 1,000 bottles.
The mistake to watch for
Mistaken answer: “The price rose, so people bought less, but the data shows more were sold, so the demand curve must have moved.”
The student mixed up two ideas and gave no cause.
The correction is to separate them. A price rise alone would move you up the demand curve with fewer bought. Here quantity also rose, so something else shifted demand, and the heatwave is the cause given in the context.
Always name the cause that the question supplies, because “demand shifted” without a reason is only a restatement of the data.
Check yourself
1. In a fictional market the price of rice fell from RM3 to RM2.50 and quantity sold rose from 600 kg to 700 kg. Which curve most likely shifted, and in which direction?
Show answer
Price down and quantity up points to a rise in supply (supply curve shifts right). A fall in demand would reduce quantity, which does not fit.
2. The price of a fictional brand of scooters rose from RM900 to RM1,000, and quantity sold fell from 400 to 350, after the cost of steel increased. Describe the chain in two sentences.
Show answer
“Price rose by about 11% while quantity fell by 12.5%, which suggests a fall in supply. The higher cost of steel raises production costs, so the supply curve shifts to the left, giving a higher equilibrium price and lower quantity.” Check: 100 ÷ 900 ≈ 11.1% and 50 ÷ 400 = 12.5%.
3. Price is unchanged and quantity sold rose. Name one possibility and say why one curve alone cannot explain it.
Show answer
Demand and supply may both have increased by amounts that left price where it was, so both curves shifted right. A shift in one curve alone would change the equilibrium price.
Where this leads next
Next, state an alternative explanation, because a chain that fits the data is not the only chain that fits. Then test yourself with the mixed practice set. The ratios tool with interpretation limits is useful when you compare figures to find the pairing.
If the chain breaks in the middle of your answer, a teacher who sees your actual written reasoning can show where. This is something we work on in online one-to-one Economics tuition.