ECO101H1 Chap.2 Markets and Elasticity
Markets and Elasticity
Demand and Supply Are Conditional Schedules
A demand schedule relates purchases to the good's own price while holding other influences fixed. A supply schedule describes sellers' planned quantities under the corresponding condition. Market analysis starts by defining the good, the period and the relevant buyers and sellers.
Without that definition, a price observation can combine unrelated products or adjustment horizons. Keep the entire schedule separate from a quantity at one price.
The distinction explains why a change in market price may cause movement along a curve without changing the willingness to buy or sell at every possible price.
Equilibrium Matches Planned Quantities
At equilibrium, quantity demanded equals quantity supplied at a common price. A shortage means buyers want more than sellers offer at the price being considered; a surplus means the reverse.
Calculate both quantities at the same price and subtract in the appropriate direction. When equations are supplied, solve the equality and substitute the answer into each schedule. The two substitution checks detect sign errors and prevent an algebraic price from being accepted when it does not reconcile the actual quantities.
The result describes the specified market under the stated assumptions.
Shocks Change Curves Through Specific Channels
A demand shift can arise from income, preferences or the prices of related goods. Whether an income rise increases demand depends on whether the good is normal or inferior for the relevant buyers. Higher input costs can reduce supply by making production more expensive.
After identifying the affected schedule, explain the response along the other schedule. With both schedules changing, separate the effects: an outcome can be determinate when both movements reinforce it, while another remains ambiguous unless the relative changes are known.
Directional reasoning should precede a confident numerical or verbal conclusion.
Elasticity Measures Proportional Sensitivity
Price elasticity compares a percentage quantity change with a percentage own-price change. It therefore differs from the slope of a graph, which is expressed in the axis units.
For a finite interval, the midpoint method uses average price and average quantity as denominators, avoiding a different magnitude when the comparison is reversed. Demand-price elasticity is frequently reported in absolute value to classify responsiveness as elastic, unit elastic or inelastic. Write whether a signed value or magnitude is intended.
A negative demand-price ratio and a positive magnitude can describe the same observed response.
Different Denominators Answer Different Questions
Cross-price elasticity measures how purchases of one good respond to another good's price. Its sign helps distinguish substitutes from complements. Income elasticity measures how purchases respond to income, distinguishing normal from inferior goods.
Supply elasticity concerns producers responding to their output price. These measures must not be interchanged simply because each is a ratio of percentages. Record the response variable in the numerator and the cause in the denominator.
Keep observations comparable across time periods and quantities; changing measurement conventions halfway through a comparison can create a response that buyers or sellers never actually made.
Revenue Connects the Price and Quantity Effects
Revenue is price multiplied by units sold. A price reduction lowers receipts on existing sales while potentially adding sales.
Elastic demand makes the proportional quantity effect dominate, whereas inelastic demand makes the price effect dominate. A straight demand curve need not have constant elasticity because the price-to-quantity ratio changes along it. Longer adjustment periods and more available substitutes can change responsiveness.
Revenue reasoning also has a boundary: higher receipts do not establish higher profit without information about costs. State the market, interval or local point and time horizon whenever interpreting an elasticity estimate.
What this chapter covers
- 01
Demand and Supply Schedules
- 02
Market Equilibrium
- 03
Shortages and Surpluses
- 04
Shifts and Movements
- 05
Midpoint Elasticity
- 06
Income and Cross-Price Elasticity
- 07
Revenue and Responsiveness
Worked example · free
Separating a Supply Shock from a Price Response
- 2Set 50 − 2P = P − 5. Initial price is 55/3 and quantity is 40/3.
- 2Set 50 − 2P = P + 4. New price is 46/3 and quantity is 58/3.
- 1Calculate the differences: price falls by 3 and quantity rises by 6.
- 2Identify the supply expansion and movement along unchanged demand; the production improvement does not itself change the demand equation.
Key terms
- Demand Schedule
- A demand schedule relates the quantity buyers want to purchase to the good’s price, holding other determinants fixed. A change in those determinants creates a different schedule.
- Market Equilibrium
- Market equilibrium is a price and quantity at which planned purchases equal planned sales. It is located by matching the two schedules rather than averaging arbitrary observed prices.
- Shortage
- A shortage is the excess of quantity demanded over quantity supplied at a specified price. Its size depends on that price and both schedules.
- Price Elasticity
- Price elasticity is the percentage quantity response divided by the percentage own-price change. Demand classifications usually compare the magnitude with one, while preserving the underlying direction separately.
- Cross-Price Elasticity
- Cross-price elasticity measures the percentage change in demand for one good relative to a percentage price change in another. The sign distinguishes substitution from complementarity in the specified comparison.
- Income Elasticity
- Income elasticity measures the percentage quantity response to a percentage income change. Positive values identify normal goods and negative values identify inferior goods for the relevant consumers.
- Total Revenue
- Total revenue is the seller’s price multiplied by units sold. It measures receipts before deducting costs, so its direction alone does not determine the change in profit.
Markets and Elasticity FAQ
Why does a higher observed price sometimes accompany more sales?
A demand expansion can raise both equilibrium price and quantity when supply remains fixed. The observations then lie on different demand schedules. They do not describe movement upward along a single unchanged demand curve, so they do not contradict the usual law of demand.
What makes the midpoint method useful for a price interval?
Using the average of the two prices and quantities gives the same elasticity magnitude when the observations are reversed. An initial-value percentage instead changes its denominator when the direction changes. Keep the midpoint denominator in both parts of the ratio.
How can a straight demand curve change elasticity?
Its slope remains constant, but elasticity also depends on the relative levels of price and quantity. Near the high-price, low-quantity end, a given quantity change is proportionally large. Lower down, the same absolute change is a smaller share of purchases.
Can an inferior good have downward-sloping demand?
Yes. Inferior describes a negative response to higher income, holding other influences fixed. Downward-sloping demand concerns the response to the good’s own price. Those are separate comparisons with different denominator variables, so the labels do not conflict.
Will a price discount always raise revenue?
A discount raises receipts only when the quantity response is sufficiently large over the relevant comparison. Elastic demand supports that result; inelastic demand does not. Even after establishing higher revenue, assess production costs separately before concluding that the discount improves profit.
Exam move
Practise a market problem in three passes. First name the shock and the determinant it changes. Next solve or draw the old and new equilibria, keeping the unaffected curve fixed. Finally explain how buyers and sellers respond at the new price. For elasticity, write the percentage numerator and denominator separately and label their variables.
Use a direct price-times-quantity calculation to check any revenue prediction when endpoints are available. Include one problem with simultaneous shifts so that an ambiguous direction is treated as a legitimate result rather than forced into a memorised arrow.
Working through Markets and Elasticity in ECO101H1? Sia is AskSia’s AI Economics tutor — ask any ECO101H1 Markets and Elasticity question and get a clear, step-by-step explanation grounded in how ECO101H1 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
