ECO 304K, Introduction to Microeconomics | University of Texas at Austin Source: Practice Mock Exam 2026 Difficulty: Introductory | Prerequisites: None
The Production Possibilities Frontier, opportunity cost, and comparative advantage form the foundation of why people and countries trade. This material appears early in the course and keeps coming back: you will use opportunity cost reasoning in consumer choice, firm behaviour, and policy analysis.
If you missed the first couple of weeks or need a refresher on why scarcity forces trade-offs, this is the place to start. The concepts here are straightforward, but the exam tests whether you can calculate and compare opportunity costs precisely.
The PPF shows the maximum combinations of two goods an economy can produce. Points on it are efficient, inside it are inefficient, outside it are unattainable (without trade). Opportunity cost is what you give up. Comparative advantage means producing at a lower opportunity cost, and it is the basis for mutually beneficial trade.
Opportunity cost
The value of the highest-valued alternative that must be sacrificed when a choice is made. In simple terms, it is what you give up to get something else.
Production Possibilities Frontier (PPF)
A curve showing the maximum combinations of two goods an economy can produce using all its resources efficiently. Also called the production possibilities curve (PPC).
Absolute advantage
The ability to produce a greater quantity of a good using the same amount of resources as another producer. Think of it as raw productivity.
Comparative advantage
The ability to produce a good at a lower opportunity cost than another producer. This is what determines who should specialise in what.
Specialisation
When a producer concentrates on producing the good in which they have a comparative advantage, rather than trying to produce everything.
Gains from trade
The increase in total output and consumption possibilities that arise when producers specialise according to comparative advantage and then trade with each other.
Law of Increasing Opportunity Cost
As an economy produces more of one good, the opportunity cost of producing each additional unit rises. This is because resources are not perfectly adaptable between uses.
Points on the frontier are productively efficient (all resources fully employed).
Points inside the frontier mean resources are underutilised or used inefficiently (unemployment, idle factories).
Points outside the frontier are currently unattainable with existing resources and technology.
The PPF shifts outward with economic growth (more resources, better technology). It does not shift because of trade.
An economy can consume outside its own PPF through trade, but it can only produce on or inside it.
As you produce more of one good, the opportunity cost of each additional unit rises.
This happens because resources are specialised: some workers, land, or equipment are better suited to producing one good than another.
When you shift resources, you start with those best suited to the new task. As you shift more, you pull in resources that were very efficient at the old task, so each extra unit costs more.
This is why the PPF is bowed outward (concave to the origin). A straight-line PPF would mean constant opportunity costs, implying perfectly adaptable resources.
To find the opportunity cost of Good X, divide the maximum output of Good Y by the maximum output of Good X.
Example from the exam: Country A produces 10 Wheat or 20 Cloth. Opportunity cost of 1 Wheat = 20/10 = 2 Cloth. Opportunity cost of 1 Cloth = 10/20 = 0.5 Wheat.
Country B produces 5 Wheat or 15 Cloth. Opportunity cost of 1 Wheat = 15/5 = 3 Cloth. Opportunity cost of 1 Cloth = 5/15 = 1/3 Wheat.
Absolute advantage: who can produce more with the same resources. Country A has absolute advantage in both Wheat (10 > 5) and Cloth (20 > 15).
Comparative advantage: who has the lower opportunity cost. Country A has comparative advantage in Wheat (2 Cloth < 3 Cloth). Country B has comparative advantage in Cloth (1/3 Wheat < 1/2 Wheat).
A country can have absolute advantage in everything but still benefit from trade by specialising in the good where its comparative advantage lies.
Each country specialises in the good where it has comparative advantage.
They trade at a rate between their respective opportunity costs.
Example: Country A specialises in Wheat (produces 10), Country B specialises in Cloth (produces 15). They trade at, say, 1 Wheat for 2.5 Cloth. Both can now consume bundles outside their individual PPFs.
Trade does not change what a country can produce, but it changes what it can consume.
Opportunity cost of Good X
OC of 1 unit of X = (Max output of Y) / (Max output of X)
Opportunity cost of Good Y
OC of 1 unit of Y = (Max output of X) / (Max output of Y)
Note: the opportunity costs of the two goods are always reciprocals of each other.
Comparative advantage explains why countries like Bangladesh export garments while importing electronics from South Korea, even though South Korea could, in principle, produce garments with fewer resources. Each country specialises where its opportunity cost is lowest, and global output rises as a result.
Students often confuse absolute advantage with comparative advantage. Absolute advantage is about total productivity. Comparative advantage is about opportunity cost. They are different things, and trade is based on comparative advantage.
"An economy can produce outside its PPF through trade" is false. It can consume outside its PPF through trade, but production stays on or inside the frontier.
The PPF bows outward because of increasing opportunity costs, not because of economic growth. Economic growth shifts the entire frontier outward.
Comparative advantage is defined by the lower opportunity cost. The description "producing with fewer resources" is absolute advantage, not comparative.
⚠️ The exam will ask you to calculate opportunity costs from a production table and identify who has comparative advantage. Get the formula down cold: OC of X = max Y / max X.
⚠️ True/false trap: "comparative advantage is the ability to produce using fewer resources." That is absolute advantage. Comparative advantage is the lower opportunity cost.
⚠️ True/false trap: "an economy can produce outside its PPF if it trades." False. It can consume outside, not produce outside.
⚠️ Know what a point inside the PPF means (inefficiency/underutilisation) and what a point outside means (unattainable).
True or false: comparative advantage means producing with fewer resources. (False)
If Country X can produce 8 cars or 24 bikes, what is the opportunity cost of 1 car? (3 bikes)
True or false: a point inside the PPF is efficient. (False, it is inefficient)
True or false: an economy can consume outside its PPF through trade. (True)
What causes the PPF to bow outward? (The law of increasing opportunity cost, because resources are specialised)
Q: Using the trade production data (Country A: 10 Wheat or 20 Cloth; Country B: 5 Wheat or 15 Cloth), calculate the opportunity cost of 1 ton of Wheat for both countries.
A: Country A: 1 Wheat = 2 Cloth (20/10). Country B: 1 Wheat = 3 Cloth (15/5).
Q: Which country has the comparative advantage in Wheat?
A: Country A, because its opportunity cost of Wheat is lower (2 Cloth vs 3 Cloth).
Q: Which country has the comparative advantage in Cloth?
A: Country B, because its opportunity cost of Cloth is lower (1/3 Wheat vs 1/2 Wheat).
Q: A point located inside the PPF indicates that...?
A: Resources are being underutilised or used inefficiently.
Q: Briefly explain the Law of Increasing Opportunity Cost and why it causes the PPF to be bowed outward.
A: As you produce more of one good, the opportunity cost of each additional unit rises because resources are specialised. You first shift the resources best suited to the new task, then eventually pull in those that were very productive in the old task. This increasing cost as you move along the frontier creates the bowed-out shape.
Q: True or false: an economy can produce at a point outside its PPF if it engages in international trade.
A: False. Trade allows an economy to consume outside its PPF, but it can only produce within or on its own frontier.
Opportunity cost is the single most recurring concept in the entire course. It comes back in consumer choice (the budget line slope is an opportunity cost ratio), in producer decisions, and in policy analysis (deadweight loss is about lost surplus from producing the wrong quantity).
Comparative advantage connects to the broader discussion of market efficiency: when producers specialise and trade, total surplus increases.
production possibilities frontier, PPF, PPC, production possibilities curve, opportunity cost, absolute advantage, comparative advantage, specialisation, gains from trade, law of increasing opportunity cost, bowed-out PPF, concave frontier, trade-offs, scarcity, resource allocation, ECO 304K, UT Austin microeconomics