The one chapter where the right answer offends common sense: a country that is worse at making everything still has something worth specialising in, and both sides end up with more. The arithmetic is short, and the exam tests it relentlessly.
Nearly every lost mark in this chapter comes from confusing these two. Get the distinction solid here and the rest is arithmetic.
| Term | Means | Measured by |
|---|---|---|
| Absolute advantage | Producing more of a good with the same resources | Output per worker-hour. Just compare the two numbers. |
| Comparative advantage | Producing a good at a lower opportunity cost | What you give up of the other good. Requires a ratio, not a comparison. |
One party can hold absolute advantage in everything. No party can hold comparative advantage in everything — it is arithmetically impossible, because opportunity costs are reciprocals. If you are the cheaper producer of laptops in forgone avocados, you are necessarily the dearer producer of avocados in forgone laptops. Trade follows comparative advantage, so there is always something for each side to do.
A surgeon may be the fastest typist in the hospital. She still does not type her own notes, because an hour of her typing costs an hour of surgery. Absolute advantage in typing, comparative advantage in surgery — and the slower typist gets the work.
This is the mechanical skill the exam actually grades. Two goods, two producers, four numbers — and a division you must do in the right direction.
For the opportunity cost of one laptop, put laptops underneath:
avocados ÷ laptops. The good you are pricing goes on the bottom. Say it out loud —
"cost of one laptop, so laptops on the bottom" — because inverting this is the single most common
error in the chapter, and it flips which country should specialise in what.
Push the four production numbers below. Notice you can give one country absolute advantage in both goods and still get a clean division of labour out of the opportunity costs.
Knowing who should make what is half the answer. The other half is the exchange rate between the two goods, and the fact that only a band of rates leaves both sides better off.
Each side specialises where its opportunity cost is lower, then swaps. The exchange rate — how many avocados buy one laptop — is the terms of trade, and it must sit between the two opportunity costs. Outside that band one side would rather make the good itself, so it refuses, exactly like the price slider in Chapter 1.
Ridge gives up 2 avocados to make a laptop, so it will not sell one for less than 2. Vale gives up 4, so it will not pay more than 4. Any rate strictly between 2 and 4 beats self-sufficiency for both. The two opportunity costs are the floor and the ceiling — that is all the band is.
But a rate inside the band is necessary, not sufficient: the volume matters too. Move both sliders and watch the "both gain" verdict flip.
Four recurring question shapes, and the error each one is hunting for.
| Asked as | Do this | Trap being set |
|---|---|---|
| "Who has absolute advantage?" | Compare raw output. No division. | Students divide anyway and answer the comparative question. |
| "Who has comparative advantage in X?" | other ÷ X for both. Lower wins. |
Inverting the ratio, which reverses the answer. |
| "Give an acceptable terms of trade" | Any number strictly between the two opportunity costs. | Quoting a rate equal to one cost — that side is indifferent, so it isn't a gain. |
| "Can both gain if one is better at everything?" | Yes. Say why: comparative advantage cannot be held in both goods. | Answering "no" from intuition. |
Shuffled rather than blocked. Several items give you a production table and nothing else — that is the exam's favourite shape.
| Model | Genuinely computed | Simplified |
|---|---|---|
| Who makes what | Both opportunity costs in both directions, absolute advantage, comparative advantage, and the terms-of-trade band — all from the four output numbers. | Constant opportunity cost (straight-line PPCs). Chapter 3 bends them. |
| The deal | Specialised output, the swap at your terms of trade, each side's final consumption, the comparison against going it alone, and the change in world output. | Two goods, two parties, no transport cost, and a fixed self-sufficiency baseline (Ridge 70L+60A, Vale 20L+80A). |