Supply Theory

IB Economics Higher Level· Unit 2: Microeconomics, Topic 2: Supply· 15 min read

1. Core Definitions and the Law of Supply★★☆☆☆⏱ 5 min

Supply describes the relationship between the price of a good and the quantity producers are willing and able to sell over a specific time period, holding all other factors constant (ceteris paribus).

📘 Definition

Law of Supply

Ceteris paribus, there is a positive (direct) relationship between the own price of a good and its quantity supplied. As price rises, quantity supplied rises; as price falls, quantity supplied falls.

Example:

If the price of coffee rises, roasters will increase the quantity of coffee supplied to the market.

The upward slope of the supply curve follows from two core producer incentives: higher prices mean higher marginal revenue per unit, and increasing output typically leads to rising marginal costs, so producers only supply more if they can charge a higher price.

📐 Worked Example

A handmade mug producer records quantity supplied at different prices: Price ( extper mug): 10, 15, 20, 25. Quantity (mugs per week): 5, 8, 11, 14. Confirm this follows the Law of Supply.

1
Draw a graph with price on the vertical axis and quantity on the horizontal axis, per standard microeconomic convention.
2
Plot each of the four points: $(5, 10), (8, 15), (11, 20), (14, 25)$
3
Connect the points to form an upward-sloping line. The upward slope confirms that as price increases, quantity supplied increases, matching the Law of Supply.

2. Movements vs Shifts of the Supply Curve★★★☆☆⏱ 4 min

This distinction is one of the most frequently tested concepts in IB microeconomics, and a common source of lost marks from incorrect terminology.

📘 Definition

Change in Quantity Supplied vs Change in Supply

A change in quantity supplied is a movement along the existing supply curve, caused only by a change in the own price of the good. A change in supply is a shift of the entire supply curve, caused by a change in one or more non-price determinants.

If supply increases (more supplied at every price), the curve shifts right. If supply decreases, it shifts left.

📐 Worked Example

For each scenario, state if there is a movement along the supply curve or a shift: (a) The price of avocados rises due to higher consumer demand. (b) A new fertilizer increases avocado yields per hectare.

1
Recall the rule: own price changes cause movement, non-price changes cause shifts.
2
Scenario (a): The change is to the own price of avocados. This causes an upward movement along the existing supply curve (increase in quantity supplied, no shift).
3
Scenario (b): The change is to production technology, a non-price determinant. This increases supply at every price, so the entire supply curve shifts right.

Exam tip:

Always confirm if the change affects own price first before deciding if it is a shift or movement.

3. Non-Price Determinants of Supply★★★☆☆⏱ 5 min

Six core categories of non-price determinants shift the market supply curve, all related to production costs or conditions:

Cost of factors of production: Lower wages, raw materials or energy costs increase supply (shift right)
Technology: Productivity improvements lower marginal costs and increase supply
Price of related goods (in production): Higher price of a production substitute decreases supply of the original good
Government intervention: Indirect taxes decrease supply; subsidies increase supply
Expectations and supply shocks: Expected future price rises can reduce current supply; natural disasters reduce supply
Number of producers: More firms entering the market increases overall market supply

📐 Worked Example

The government introduces a ext2 per unit subsidy on electric vehicles. What is the impact on the market supply curve?

1
Identify the type of determinant: a subsidy is government intervention that lowers per-unit production costs for producers.
2
Lower production costs mean producers are willing to supply a larger quantity of electric vehicles at every price point.
3
Conclusion: The entire market supply curve for electric vehicles shifts to the right, with no movement along the original curve.

4. Linear Supply Functions (HL Only)★★★☆☆HL only⏱ 4 min

IB HL requires you to interpret, plot, and calculate values from linear supply functions, commonly used in Paper 2 data response questions.

📘 Definition

Linear Supply Function

Q_s = c + dP

$Q_{s}$ = quantity supplied, $P$ = own price, $c$ = constant intercept term, $d$ = slope of the function. The slope $d$ is always positive for a supply function following the Law of Supply.

📐 Worked Example

Given the supply function $Q_{s} = - 10 + 2 P$ , calculate quantity supplied when $P = 15$ , and find the price required to get a quantity supplied of 20 units.

1
Substitute $P = 15$ into the function to find quantity supplied:
2
$Q_s = -10 + (2 \times 15) = -10 + 30 = 20$
3
Substitute $Q_{s} = 20$ and rearrange to solve for the required price:
4
$20 = -10 + 2P \implies 30 = 2P \implies P = 15$
5
Confirm the positive slope matches the Law of Supply, which it does here ( $d = 2 > 0$ ).

5. Common Pitfalls

Wrong move:

Drawing an increase in supply as an upward shift instead of a right shift

Why:

Upward sloping curves lead students to associate 'more' with upward movement, but quantity is measured on the horizontal axis

Correct move:

An increase in supply (more at every price) always shifts the curve right; a decrease shifts it left

Wrong move:

Calling an own-price movement a 'shift in supply'

Why:

Examiners explicitly test terminology distinction and deduct marks for incorrect wording

Correct move:

Use 'change in quantity supplied' for movements, 'change in supply' for shifts

Wrong move:

Assuming all related good price changes shift supply left

Why:

Students forget that related goods can be complements in production, not just substitutes

Correct move:

For production complements (e.g. beef and leather), a price increase for beef increases supply of leather, shifting it right

Wrong move:

Using a negative slope for a linear supply function

Why:

Confusing supply functions with negative-sloping demand functions

Correct move:

The slope coefficient in a standard linear supply function $Q_{s} = c + d P$ is always positive

Wrong move:

Rejecting a linear supply function because the intercept is negative

Why:

Students expect all intercepts to be positive

Correct move:

A negative intercept just means producers will not supply any output until price rises above a minimum level, which is realistic

6. Quick Reference Cheatsheet

Concept	Cause	Type of Change	Curve Change
Change in Quantity Supplied	Change in own price	Change in Qs	Movement along curve
Increase in Supply	Lower costs / better tech / subsidy	Increase in supply at all P	Shift right
Decrease in Supply	Higher costs / tax / supply shock	Decrease in supply at all P	Shift left
Linear Supply Function (HL)	N/A	N/A	$Q_{s} = c + d P, d > 0$

7. Frequently Asked

What is the key difference between a shift and movement along the supply curve?

A movement along the curve is only caused by a change in the own price of the good, and changes quantity supplied. A shift of the entire curve is caused by a change in a non-price determinant, and changes overall supply at every price.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · Paper 1
10 mark question on supply shifts
2024 · Paper 2
Data response on agricultural supply
2023 · Paper 1
Explain difference between shift and movement

Going deeper

syllabusIB Economics HL 2020 SyllabusOfficial unit reference

What's Next

Supply theory is the foundation for understanding market equilibrium, price determination, and government intervention, which make up over 30% of the marks on IB Economics HL exams. Mastering the distinction between shifts and movements is critical for correctly analyzing almost all microeconomic policy questions, from price controls to indirect taxation and international trade. Once you are comfortable with core supply theory, you can move on to combining supply and demand to analyze full market outcomes, and extend your knowledge to elasticity of supply.