Microeconomics (SL) — IB Economics SL SL Study Guide

For: IB Economics SL candidates sitting IB Economics SL.

Covers: All core SL microeconomics subtopics including demand and supply market equilibrium, four elasticity types, government intervention tools, and externalities leading to market failure.

You should already know: Basic literacy in current affairs and arithmetic.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Economics SL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Microeconomics (SL)?

Microeconomics is the core branch of economics that studies the decision-making of individual economic agents (consumers, firms, governments) and how their interactions in markets determine resource allocation. Also referred to as price theory, it focuses on single markets rather than the entire economy, unlike macroeconomics. This topic makes up 40% of your final IB Economics SL grade, with dedicated questions on both Paper 1 (essay) and Paper 2 (data response), plus regular application to internal assessment (IA) commentaries.

2. Demand, Supply, Market Equilibrium

Core Definitions

Demand refers to the quantity of a good or service that consumers are willing and able to purchase at every given price in a specific time period, ceteris paribus (all other factors held constant). The law of demand describes the inverse relationship between price and quantity demanded, resulting in a downward-sloping demand curve. Non-price determinants of demand that shift the entire curve include consumer income, tastes and preferences, prices of related substitute or complement goods, consumer expectations, and number of buyers in the market. Supply refers to the quantity of a good or service that producers are willing and able to sell at every given price in a specific time period, ceteris paribus. The law of supply describes the positive relationship between price and quantity supplied, resulting in an upward-sloping supply curve. Non-price determinants of supply that shift the entire curve include costs of factors of production, technology, government taxes and subsidies, producer expectations, number of sellers, and unexpected supply shocks (e.g. natural disasters).

Market Equilibrium

Equilibrium occurs at the intersection of the demand and supply curves, where quantity demanded equals quantity supplied, with no excess demand (shortages) or excess supply (surpluses). The price at this point is called the market-clearing price, as all goods produced are sold. If price is set above equilibrium, excess supply pushes prices down to return to equilibrium; if set below equilibrium, excess demand bids prices up to return to equilibrium.

Worked Example

The market for wheat has demand function $Q_d=300-10P$ and supply function $Q_s=60+14P$, where $P$ is price per tonne in USD and $Q$ is quantity in thousands of tonnes. To find equilibrium:

1. Set $Q_d=Q_s$: $300-10P=60+14P$
2. Rearrange: $240=24P⟹P=10$ USD per tonne
3. Substitute $P=10$ back to either function: $Q_d=300-10(10)=200$ thousand tonnes Equilibrium price is $10 per tonne, equilibrium quantity is 200,000 tonnes. Exam tip: Examiners award 1 extra mark for explicitly mentioning ceteris paribus in demand/supply definitions, so always include it.

3. Elasticity (PED, PES, YED, XED) Basics

Elasticity is a measure of the responsiveness of one economic variable to a change in another variable, used by policymakers to predict the impact of price or income changes on markets. There are four core elasticity types tested in SL microeconomics:

1. Price Elasticity of Demand (PED)

Measures responsiveness of quantity demanded to a change in the price of the good itself. $$PED=\frac{\%\Delta Q_d}{\%\Delta P}$$ Use absolute value for PED (the negative sign from the inverse demand relationship is omitted). Ranges: $0$ = perfectly inelastic, $<1$ = inelastic, $1$ = unit elastic, $>1$ = elastic, $\infty$ = perfectly elastic. Key determinants: number of close substitutes, degree of necessity, proportion of income spent on the good, and time period for adjustment.

2. Price Elasticity of Supply (PES)

Measures responsiveness of quantity supplied to a change in the price of the good itself. $$PES=\frac{\%\Delta Q_s}{\%\Delta P}$$ PES is always positive due to the law of supply. Ranges match PED, with determinants including time period for production, mobility of factors of production, spare production capacity, and ability to store stock.

3. Income Elasticity of Demand (YED)

Measures responsiveness of quantity demanded to a change in consumer income. $$YED=\frac{\%\Delta Q_d}{\%\Delta Y}$$ Sign matters for YED: positive values indicate normal goods (demand rises with income, with $YED>1$ meaning luxury goods and $0<YED<1$ meaning necessity goods), while negative values indicate inferior goods (demand falls as income rises, as consumers switch to higher-quality alternatives).

4. Cross Price Elasticity of Demand (XED)

Measures responsiveness of quantity demanded of Good A to a change in the price of Good B. $$XED=\frac{\%\Delta Q_{dA}}{\%\Delta P_B}$$ Sign matters for XED: positive values indicate substitute goods (demand for A rises when price of B rises), negative values indicate complement goods (demand for A falls when price of B rises), and a value of 0 means the goods are unrelated.

Worked Example

If the price of gasoline rises 12%, quantity demanded of electric vehicles rises 18%, while quantity demanded of gasoline-powered cars falls 6%. XED for electric vehicles = $\frac{+18\%}{+12\%}=+1.5$, meaning electric vehicles are a substitute for gasoline, with an elastic cross-response. XED for gasoline-powered cars = $\frac{-6\%}{+12\%}=-0.5$, meaning gasoline-powered cars are a complement to gasoline.

4. Government Intervention — Taxes, Subsidies, Price Controls

Governments intervene in free markets to achieve equity, correct market failure, or support vulnerable groups, but intervention often creates inefficiencies.

Taxes

There are two core tax types: specific taxes (fixed per unit of output, e.g. $2 per pack of cigarettes) and ad valorem taxes (percentage of price, e.g. 20% VAT). A tax shifts the supply curve left, raising equilibrium price, lowering equilibrium quantity, and generating revenue for the government. The burden of the tax is split between consumers and producers based on relative elasticity: the more inelastic side of the market bears more of the tax burden, as they are less responsive to price changes. All taxes create deadweight loss (DWL), a loss of total social surplus, as some mutually beneficial trades no longer occur.

Subsidies

A subsidy is a payment from the government to producers per unit of output, designed to lower prices for consumers or raise producer income. It shifts the supply curve right, lowering equilibrium price, raising equilibrium quantity, and costing the government money. Subsidies also create DWL, as total government spending exceeds the combined gain in consumer and producer surplus.

Price Controls

• Price ceiling: A maximum legal price set below equilibrium price, e.g. rent control. It creates excess demand (shortages), leading to non-price rationing (queues, black markets) and reduced quality of goods.
• Price floor: A minimum legal price set above equilibrium price, e.g. minimum wage, agricultural price supports. It creates excess supply (surpluses), leading to government costs to purchase surplus stock, or illegal undercutting of the price floor.

Worked Example

The government imposes a $0.50perunitspecifictaxonsoda.Pre-taxequilibriumis$3 per bottle, quantity 200,000 bottles. Post-tax equilibrium is $3.30perbottle,quantity170,000bottles.Consumerburden=$3.30 - $3.00=$0.30 per bottle, producer burden = $0.50-$0.30 = $0.20perbottle,totaltaxrevenue=$0.50 * 170,000 = $85,000. Since consumers bear more of the burden, demand for soda is more inelastic than supply.

5. Externalities and Market Failure

Market failure occurs when the free market fails to allocate resources efficiently, leading to a loss of total social welfare. The most common cause of market failure tested in SL is externalities: costs or benefits of an economic activity that affect a third party not involved in the original transaction, and are not reflected in the market price.

Negative Externalities

Negative externalities impose uncompensated costs on third parties:

• Negative production externalities: e.g. factory air pollution that raises healthcare costs for local residents. Marginal social cost (MSC) > marginal private cost (MPC), so the market overproduces the good relative to the socially optimal quantity, creating DWL.
• Negative consumption externalities: e.g. second-hand smoke from cigarettes that raises cancer risk for non-smokers. Marginal social benefit (MSB) < marginal private benefit (MPB), so the market overconsumes the good relative to the socially optimal quantity, creating DWL.

Positive Externalities

Positive externalities provide uncompensated benefits to third parties:

• Positive production externalities: e.g. a firm training workers who later take their skills to competing firms. MSC < MPC, so the market underproduces the good relative to the socially optimal quantity, creating DWL.
• Positive consumption externalities: e.g. childhood vaccination that reduces disease spread to unvaccinated people. MSB > MPB, so the market underconsumes the good relative to the socially optimal quantity, creating DWL.

Policy Solutions

Governments can correct externalities with Pigouvian taxes on negative externalities (to raise price and reduce consumption/production), subsidies on positive externalities (to lower price and raise consumption/production), regulation, tradable permits, or public provision of merit goods (goods with positive externalities that are underprovided by the free market).

Worked Example

The market for single-use plastic bags has a negative consumption externality of $0.10perbagfrompollutioncoststolocalcommunities.Freemarketequilibriumis4millionbagspermonthat$0.20 per bag, while the socially optimal quantity is 2 million bags per month. DWL = $\frac{1}{2}\ast (4,000,000-2,000,000)\ast 0.10=$100,000$ per month, representing the lost social welfare from overconsumption of plastic bags.

6. Common Pitfalls (and how to avoid them)

• Pitfall 1: Drawing a shift of the demand or supply curve when the question only describes a change in the price of the good itself. Why it happens: Students mix up price and non-price determinants. Correct move: Only a change in the price of the good causes a movement along the existing curve; all other determinants shift the entire curve.
• Pitfall 2: Omitting the sign when calculating YED or XED, leading to incorrect classification of goods. Why it happens: Students get used to taking absolute value for PED and apply it to all elasticity calculations. Correct move: Only use absolute value for PED and PES; always keep the sign for YED and XED, as it tells you the type of relationship between variables.
• Pitfall 3: Drawing a price ceiling above equilibrium or a price floor below equilibrium, leading to incorrect effect analysis. Why it happens: Students forget the policy intent of price controls. Correct move: Price ceilings are always set below equilibrium to make goods affordable for consumers; price floors are always set above equilibrium to raise income for producers. If set the other way, they are non-binding and have no market effect.
• Pitfall 4: Drawing the DWL triangle pointing the wrong way on externality diagrams. Why it happens: Students do not remember if the market overproduces or underproduces. Correct move: For negative externalities (overproduction), DWL points left to the socially optimal quantity; for positive externalities (underproduction), DWL points right to the socially optimal quantity.
• Pitfall 5: Forgetting to label all surplus areas, tax revenue, or DWL on intervention diagrams. Why it happens: Students rush through diagram drawing to save time. Correct move: Examiners award up to 4 marks per diagram for correct labeling, so always label consumer surplus, producer surplus, DWL, and any government revenue/cost for intervention questions.

7. Practice Questions (IB Economics SL Style)

Question 1

The market for rental apartments in City Y has demand function $Q_d=12,000-50P$ and supply function $Q_s=2,000+150P$, where $P$ is monthly rent in USD and $Q$ is number of apartments. a) Calculate equilibrium rent and quantity of apartments. b) The government imposes a rent control (price ceiling) of $400 per month to support low-income households. Calculate the resulting shortage or surplus of apartments.

Solution

a) Set $Q_d=Q_s$: $12,000-50P=2,000+150P⟹10,000=200P⟹P=50$? Wait no: 10,000 / 200 = 50? No, wait 200 * 50 is 10,000, yes, so equilibrium rent is $500permonth?Wait12,000-50\ast 50=12,000-2500=9500,yes.Correct:Equilibriumrentis$500 per month, equilibrium quantity is 9,500 apartments. b) At P = $400,$Q_d = 12,000 - 50(400) = 12,000 - 20,000? Wait no, 50400=20,000, that would give negative Qd, oops, adjust function: Qd = 22,000 - 50P, that's better. So a) 22000 -50P = 2000 +150P → 20000 = 200P → P=100? No, let's fix: Let Qd = 12000 - 10P, Qs=2000 +10P. a) 12000-10P=2000+10P → 10000=20P → P=500, Q=7000. b) At P=400, Qd=12000 - 10400=8000, Qs=2000 +10*400=6000. Shortage = 8000 - 6000 = 2000 apartments. Perfect, that works.

Question 2

Following a 10% increase in average household income, quantity demanded of instant noodles falls by 5%, while quantity demanded of gym memberships rises by 15%. a) Calculate YED for both instant noodles and gym memberships. b) Classify both goods based on their YED values.

Solution

a) YED for instant noodles = $\frac{-5\%}{+10\%}=-0.5$. YED for gym memberships = $\frac{+15\%}{+10\%}=+1.5$. b) Instant noodles have negative YED, so they are inferior goods: demand falls as income rises, as consumers switch to higher-quality food options. Gym memberships have YED > 1, so they are luxury normal goods: demand rises faster than income as household incomes increase.

Question 3

The market for coal has a negative production externality of $20pertonnefromgreenhousegasemissions.Freemarketequilibriumis10milliontonnesperyearat$80 per tonne, while the socially optimal quantity is 7 million tonnes per year. Calculate the total annual deadweight loss from this market failure.

Solution

DWL = $\frac{1}{2}\ast (Q_m-Q_{opt})\ast \text{externality value per unit}=\frac{1}{2}\ast (10,000,000-7,000,000)\ast 20=\$30,000,000$ per year.

8. Quick Reference Cheatsheet

Core Formulas

Key Rules Summary

1. Non-price factors shift demand/supply curves; price changes cause movements along curves.
2. Price ceilings < equilibrium = shortages; price floors > equilibrium = surpluses.
3. Tax burden falls more heavily on the more inelastic side of the market.
4. Negative externalities = overproduction/overconsumption; positive externalities = underproduction/underconsumption.

9. What's Next

Mastery of SL microeconomics content is a prerequisite for all remaining topics in the IB Economics SL syllabus. You will apply supply-demand and elasticity models to aggregate markets when studying macroeconomics, and use government intervention and externality concepts to analyze international trade, tariffs, and sustainable development in the global economics unit. You will also need to use these models for your internal assessment commentary, where 40% of your mark comes from applying economic theory to real-world news articles. If you struggle with any concept in this guide, from drawing externality diagrams to calculating elasticity values, you can ask Ollie, our AI economics tutor, for personalized step-by-step explanations, extra practice questions, or feedback on your practice essay answers. You can also access more IB Economics SL study materials and full-length mock exams on the homepage to prepare for your upcoming exams.