ECON8069 Chap.2 Models, Markets and Equilibrium
Models, Markets and Equilibrium
A useful market model starts by defining the commodity, agents and rules. The chapter uses that boundary to distinguish movements along a curve from shifts caused by non-price determinants, and it solves equilibrium by matching intended purchases with intended sales.
Price ceilings and floors are then tested against the unconstrained equilibrium so that a policy label is never mistaken for a behavioural effect.
Define the object before the graph
A commodity can be narrow or broad, agents can enter or leave the relevant area, and regulation can change the rules under which exchange occurs. The right boundary depends on the business or policy question.
A pricing decision for one brand may require a narrow substitute set, while a national shortage analysis may treat a broader category as the good.
Give shifts a cause
A change in the good's own price produces a movement along demand or supply. Income, tastes, input prices, technology and the number of market participants can shift an entire schedule.
The observed price and quantity changes are equilibrium consequences, so they cannot by themselves identify a curve without a causal determinant.
Use equilibrium as a consistency condition
Competitive equilibrium matches intended purchases with intended sales at one price. A price below equilibrium creates excess demand, while a price above equilibrium creates excess supply.
The adjustment story explains why the crossing matters: imbalances create pressure for changed offers, bids or quantities under the model's rules.
Test policies against the baseline
A maximum price matters only below equilibrium, and a minimum price matters only above it. When a rule binds, calculate intended demand and intended supply at the legal price.
Their difference is the imbalance, but actual trade is constrained by the short side of the market and by the allocation mechanism.
What this chapter covers
- 01
Tractable, deductive and falsifiable economic models
- 02
Commodity, agents and trading rules
- 03
Market boundaries for a business question
- 04
Movements along curves versus curve shifts
- 05
Competitive equilibrium and adjustment pressure
- 06
Binding and non-binding price controls
Worked example · free
Solving equilibrium and testing a ceiling
- 2Equate demand and supply to obtain 80 = 5P, so equilibrium price is $16.
- 1Substitute $16 into either equation to obtain equilibrium quantity 42.
- 3At $12, demand is 54 and supply is 34, so the binding ceiling creates a shortage of 20 units.
Key terms
- Market Boundary
- The commodity, agents, place, horizon and rules included in one analysis.
- Demand Shift
- A change in desired quantity at every price caused by a non-price determinant.
- Excess Demand
- A shortage in which intended purchases exceed intended sales at the stated price.
- Binding Ceiling
- A maximum legal price below equilibrium that changes the market outcome.
Models, Markets and Equilibrium FAQ
How do I identify which curve shifted?
Find the changed non-price determinant first. Then move only the curve linked to that determinant and read both new equilibrium coordinates. The observed price is normally an outcome of the shift, not the reason the curve moved.
Why can the same product belong to different markets?
The relevant boundary depends on the decision. One brand may face close substitutes in a retail pricing question, while a national policy question may define the commodity more broadly and include different agents and rules.
Does every price ceiling create a shortage?
No. A ceiling changes behaviour only when it is below the unconstrained equilibrium price. A ceiling above equilibrium permits the market-clearing price, so intended demand and supply still match.
What should I verify after solving equilibrium?
Substitute the candidate price into both schedules and confirm that they produce the same feasible quantity. Then compare any policy price with that baseline before calculating a shortage or surplus, because a non-binding rule leaves the equilibrium unchanged.
Exam move
For every market question, write the commodity, agents and rule in one line. Mark whether the prompt changes the good's own price or a non-price determinant. When solving equations, verify equilibrium by substituting into both demand and supply. For a control, prove that it binds before measuring the imbalance.
Create a sign table for the four basic single-curve shifts and test a proposed explanation against both price and quantity. When the signs do not fit, state that the shock is wrong or that more than one curve moved. For model evaluation, add one sentence on tractability, one observation that could falsify the prediction and one assumption that limits application.
A neatly solved crossing is not enough when the market boundary or adjustment mechanism does not fit the case. After each diagram, narrate the causal sequence aloud: changed determinant, shifted schedule, temporary imbalance, adjustment and new equilibrium. Redraw the same shock with steeper curves to confirm that the direction survives even though the size of the price and quantity responses changes.
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