Absolute and Comparative Advantage

Absolute advantage just counts output: whoever makes more of a good with the same inputs has it. Comparative advantage counts tradeoffs: whoever gives up less of the other good to make one unit has it. A lawyer might type faster than her assistant in absolute terms, but every minute she spends typing is a minute she isn't earning lawyer fees, which are worth far more than typing. Her opportunity cost of typing is huge, so she should let the assistant type even though she's objectively faster at it.

To find comparative advantage, calculate each producer's opportunity cost for each good: divide what they give up by what they gain. Whoever has the lower opportunity cost for a good should specialize in that good. Because opportunity costs for two goods are mirror images of each other (flip one fraction to get the other), a producer with a low opportunity cost in good A automatically has a high one in good B. This guarantees that comparative advantage splits neatly between two producers, one per good, even if one producer is better at everything in absolute terms.

This sets up the next step in the unit: once you know who should specialize in what, the production possibilities frontier shows the gains from trading along comparative advantage. Total output rises above what either producer could manage alone, which is the whole case for specialization and trade rather than self-sufficiency.