Market Equilibrium and Price Changes

Q: How do I structure a 4-mark equilibrium explanation answer?

1. State the non-price factor causing the demand/supply shift, 2. Identify the direction of the curve shift, 3. Explain the resulting excess demand/supply at the original price, 4. State the final effect on equilibrium price and quantity.

Economics· 2.5, 2.6· 25 min read

1. Defining and Calculating Market Equilibrium★★☆☆☆⏱ 7 min

📘 Definition

Market Equilibrium

$D = S$ at $P_{e}, Q_{e}$

The stable market state where quantity demanded equals quantity supplied, with no inherent pressure for price to change.

Example:

If 100 coffees are demanded at $2 and 100 coffees are supplied at $2, $2 is the equilibrium price and 100 is equilibrium quantity.

At equilibrium, the market clears: there is no unsold leftover stock (excess supply) and no unmet consumer demand (excess demand). You can calculate equilibrium values from a demand/supply schedule, or from a diagram where the demand and supply curves intersect.

📐 Worked Example

Use the schedule below to find the equilibrium price and quantity of printed t-shirts:

Price ($)	Quantity Demanded	Quantity Supplied
5	200	50
10	150	100
15	100	100
20	50	150

1
Find the row where quantity demanded matches quantity supplied.
2
At a price of $15, both quantity demanded and quantity supplied equal 100 units.
3
State the final values: equilibrium price = $15, equilibrium quantity = 100 t-shirts.

Exam tip:

When drawing equilibrium diagrams, add dotted lines from the intersection point to both axes, and label the equilibrium point $E$ to earn full marks for diagram work.

2. Excess Supply and Excess Demand (Disequilibrium)★★★☆☆⏱ 6 min

📘 Definition

Disequilibrium

An unstable market state where quantity demanded does not equal quantity supplied, creating pressure for price to adjust until equilibrium is restored.

When price is set above equilibrium, quantity supplied exceeds quantity demanded, creating excess supply (surplus). Producers will cut prices to clear unsold stock, increasing quantity demanded and reducing quantity supplied until equilibrium is restored. When price is set below equilibrium, quantity demanded exceeds quantity supplied, creating excess demand (shortage). Consumers bid up prices to access limited stock, reducing quantity demanded and increasing quantity supplied until equilibrium is restored.

📐 Worked Example

Using the t-shirt schedule from the previous example, identify the type of disequilibrium at a price of $20, and explain how the market will adjust back to equilibrium.

1
Extract values at $20: quantity demanded = 50, quantity supplied = 150.
2
Quantity supplied > quantity demanded, so there is excess supply of 100 t-shirts.
3
Producers reduce prices to sell unsold stock. As price falls, quantity demanded rises and quantity supplied falls, until price reaches $15 where quantity demanded = quantity supplied = 100.

Exam tip:

Always calculate the size of the surplus or shortage (difference between quantity supplied and demanded) when asked, as this earns an extra mark in structured questions.

3. Effects of Demand Shifts on Equilibrium★★★☆☆⏱ 6 min

Any non-price demand shift factor (e.g. changes in income, consumer tastes, price of related goods, population) will shift the entire demand curve, changing the equilibrium point. A rightward (increase) in demand raises both equilibrium price and quantity traded. A leftward (decrease) in demand reduces both equilibrium price and quantity traded.

📐 Worked Example

The market for bicycles is initially in equilibrium. Explain how a successful government campaign promoting cycling for exercise will affect equilibrium price and quantity of bicycles.

1
The campaign changes consumer tastes in favour of cycling, increasing demand for bicycles at every price.
2
The demand curve shifts right from $D_{1}$ to $D_{2}$ .
3
At the original equilibrium price, there is now excess demand for bicycles.
4
Price rises until a new equilibrium is reached at a higher price ( $P_{2}$ ) and higher quantity traded ( $Q_{2}$ ).

4. Effects of Supply Shifts on Equilibrium★★★☆☆⏱ 6 min

Any non-price supply shift factor (e.g. changes in production costs, technology, number of producers, government taxes/subsidies) will shift the entire supply curve, changing the equilibrium point. A rightward (increase) in supply reduces equilibrium price and raises equilibrium quantity traded. A leftward (decrease) in supply raises equilibrium price and reduces equilibrium quantity traded.

📐 Worked Example

The market for wheat is initially in equilibrium. A global drought reduces wheat harvests across all major producing countries. Explain the effect on equilibrium price and quantity of wheat.

1
The drought reduces the amount of wheat that can be produced at every price, decreasing supply of wheat.
2
The supply curve shifts left from $S_{1}$ to $S_{2}$ .
3
At the original equilibrium price, there is now excess demand for wheat.
4
Price rises until a new equilibrium is reached at a higher price ( $P_{2}$ ) and lower quantity traded ( $Q_{2}$ ).

Exam tip:

When drawing shift diagrams, label original curves $D_{1} / S_{1}$ and new curves $D_{2} / S_{2}$ clearly, plus original equilibrium $E_{1}$ and new equilibrium $E_{2}$ to avoid losing marks.

5. Common Pitfalls

Wrong move:

Confusing movements along curves with curve shifts when explaining price changes

Why:

Price changes cause movements along existing D/S curves, only non-price factors cause shifts. Citing price as a shift cause loses marks.

Correct move:

First state the non-price factor causing the D/S shift, then explain the resulting price change as the effect of that shift.

Wrong move:

Mixing up equilibrium changes when supply shifts left

Why:

Many students incorrectly assume a leftward supply shift leads to lower price, but reduced supply makes goods scarcer so price rises.

Correct move:

Leftward supply shifts = higher price, lower quantity; rightward supply shifts = lower price, higher quantity.

Wrong move:

Skipping the disequilibrium adjustment step in explanations

Why:

Mark schemes explicitly award marks for explaining the excess demand/supply stage between old and new equilibrium.

Correct move:

After stating the curve shift, explain the excess demand/supply at the original price, then describe how price adjusts to the new equilibrium.

Wrong move:

Labelling diagram axes incorrectly

Why:

Swapping axes or using non-standard labels (e.g. 'Cost' instead of 'Price') loses all marks for diagram work.

Correct move:

Always label the y-axis $P$ (Price) and x-axis $Q$ (Quantity) before drawing any curves.

Wrong move:

Assuming both demand and supply shift unless explicitly stated

Why:

Nearly all IGCSE questions ask you to analyse the effect of a single shift (only D or only S) unless told otherwise.

Correct move:

Only analyse shifts in both curves if the question explicitly states factors affecting both demand and supply are changing.

6. Quick Reference Cheatsheet

Market Change	Curve Shift Direction	Effect on $P_{e}$	Effect on $Q_{e}$
Increase in Demand	Demand shifts right	Rises	Rises
Decrease in Demand	Demand shifts left	Falls	Falls
Increase in Supply	Supply shifts right	Falls	Rises
Decrease in Supply	Supply shifts left	Rises	Falls
Price above equilibrium	No curve shift	Falls to $P_{e}$	Adjusts to $Q_{e}$
Price below equilibrium	No curve shift	Rises to $P_{e}$	Adjusts to $Q_{e}$

7. Frequently Asked

Do I need to draw equilibrium diagrams for the exam?

Yes, Paper 2 structured questions often require fully labelled D/S diagrams showing equilibrium shifts. Label axes $P$ (Price) and $Q$ (Quantity), original/new curves, and original/new equilibrium points to earn full marks.

How do I structure a 4-mark equilibrium explanation answer?

State the non-price factor causing the demand/supply shift, 2. Identify the direction of the curve shift, 3. Explain the resulting excess demand/supply at the original price, 4. State the final effect on equilibrium price and quantity.

Going deeper

official resourceCIE IGCSE Economics 0455 SyllabusOfficial 2023-2026 syllabus document

What's Next

Now that you have mastered market equilibrium analysis, you can apply this knowledge to understand government interventions in markets, such as price controls, taxes and subsidies, which are core topics in the rest of the Allocation of Resources unit. Practice drawing equilibrium shift diagrams for different real-world scenarios, as these are frequently tested in Paper 2 structured questions worth 4-6 marks each. Make sure you can explain every step of the adjustment process clearly, as this is where most students lose marks in exam answers. Next, move on to learn about government market interventions and their effects.