AP® Microeconomics · Unit 1 · Topic 1.4
Output Problems and Input Problems
Output problems give totals a country can produce, so opportunity cost is the other good's total over this good's total. Input problems give time or resources needed per unit, so opportunity cost is this good's input over the other good's input — the ratio flips.
Comparative advantage always compares opportunity costs, but the arithmetic depends on what the numbers in the problem actually measure. An output problem tells you how much a country can produce with a fixed stock of resources: Alba can make 8 chairs OR 4 tables. More output of one good is good, so to find the cost of a chair in tables, you ask what you gave up — divide the OTHER good's total by THIS good's total: 4 tables / 8 chairs = 0.5 tables per chair.
An input problem tells you the opposite kind of fact: how much labor or time one unit costs. A shirt takes 5 hours, a hat takes 2 hours. Here more is bad — more hours means more cost, not more output. So the ratio direction flips: to find the opportunity cost of a shirt in hats, divide THIS good's input by the OTHER good's input: 5 hours / 2 hours = 2.5 hats per shirt. Mixing up which kind of problem you're in and applying the wrong ratio is the single most common error in these questions.
This matters because every comparative advantage and specialization question downstream — who should produce what, where the terms of trade can fall — depends on getting opportunity costs right first. Before dividing anything, read the units: totals of goods produced mean output problem, hours or resources per unit mean input problem. Get that classification right and the rest of the unit's trade reasoning follows cleanly.
Key terms
- Output problem
- A setup giving total units of each good a country can produce with its resources.
- Input problem
- A setup giving the hours or resources needed to produce one unit of each good.
- Opportunity cost
- The amount of one good given up to produce one more unit of another good.
- Comparative advantage
- The ability to produce a good at a lower opportunity cost than another producer.
Watch it explained
- How it worksHow do you tell an output problem from an input problem before you divide anything?
- Worked exampleFind each country's opportunity cost per chair when Alba makes 8 chairs or 4 tables and Bota makes 6 chairs or 6 tables.
- Worked exampleFind comparative advantage when a shirt takes 5 hours and a hat 2 hours in one country, and 9 and 3 in another.
- Common mistakeWhy isn't the smaller number always the one with the advantage in an input problem?
Check yourself
End of topic.