# Supply Theory

> IB Economics Higher Level · IB Diploma Programme Economics
> Source: https://www.owlsprep.com/study/ib-economics-hl-u2-supply-theory/

This module covers core supply theory for IB Economics HL, including the law of supply, non-price determinants of supply, the critical distinction between shifts and movements along the supply curve, and linear supply function calculations.

**Prerequisites:** [Basic demand theory](https://www.owlsprep.com/study/ib-economics-hl-u2-demand-theory/); [Ceteris paribus assumption](https://www.owlsprep.com/study/ib-economics-hl-u1-basic-economic-concepts/)

## Learning objectives

- Define supply and the law of supply
- Explain the difference between movement along and shift of the supply curve
- Identify and analyze non-price determinants of supply
- Interpret and calculate values from linear supply functions
- Apply supply theory to IB exam questions

## Core Definitions and the Law of Supply

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).

**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 (\textper 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.

## Movements vs Shifts of the Supply Curve

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

**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.

## Non-Price Determinants of Supply

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 \text2 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.

## Linear Supply Functions (HL Only)

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

**Linear Supply Function** — $Q_s$ = 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.

*Notation:* Q_s = c + dP

**Worked example:** Given the supply function $Q_s = -10 + 2P$, 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 ($d=2>0$).

## Common pitfalls

- **Wrong:** Drawing an increase in supply as an upward shift instead of a right shift
  - Why it fails: Upward sloping curves lead students to associate 'more' with upward movement, but quantity is measured on the horizontal axis
  - Correct: An increase in supply (more at every price) always shifts the curve right; a decrease shifts it left
- **Wrong:** Calling an own-price movement a 'shift in supply'
  - Why it fails: Examiners explicitly test terminology distinction and deduct marks for incorrect wording
  - Correct: Use 'change in quantity supplied' for movements, 'change in supply' for shifts
- **Wrong:** Assuming all related good price changes shift supply left
  - Why it fails: Students forget that related goods can be complements in production, not just substitutes
  - Correct: For production complements (e.g. beef and leather), a price increase for beef increases supply of leather, shifting it right
- **Wrong:** Using a negative slope for a linear supply function
  - Why it fails: Confusing supply functions with negative-sloping demand functions
  - Correct: The slope coefficient in a standard linear supply function $Q_s = c + dP$ is always positive
- **Wrong:** Rejecting a linear supply function because the intercept is negative
  - Why it fails: Students expect all intercepts to be positive
  - Correct: A negative intercept just means producers will not supply any output until price rises above a minimum level, which is realistic

## 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 | $Q_s = c + dP, d>0$ |

## 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.

- [Competitive market equilibrium](https://www.owlsprep.com/study/ib-economics-hl-u2-competitive-market-equilibrium/)
- [Consumer and producer surplus](https://www.owlsprep.com/study/ib-economics-hl-u2-consumer-and-producer-surplus/)
- [Government intervention in markets](https://www.owlsprep.com/study/ib-economics-hl-u2-government-intervention-in-markets/)

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