Microeconomics · Unit 1: Basic Economic Concepts · 18 min read · Updated 2026-05-11
Basic Economic Concepts — AP Microeconomics
AP Microeconomics · Unit 1: Basic Economic Concepts · 18 min read
1. Scarcity, Opportunity Cost, and the PPC Model★☆☆☆☆⏱ 5 min
Scarcity is the fundamental problem of economics: all productive resources (land, labor, capital, entrepreneurship) are finite, but human wants are infinite, meaning trade-offs are always unavoidable. Every choice has an opportunity cost, defined as the value of the single next-best alternative you give up, not the sum of all possible alternatives.
Key properties of the PPC model:
Points on the PPC: productively efficient (no more of one good can be produced without reducing output of the other)
Points inside the PPC: inefficient (unused or underemployed resources)
Points outside the PPC: unattainable with current resources and technology
Straight-line PPC: constant opportunity cost (resources equally suited to both goods)
Bowed-out (concave) PPC: increasing opportunity cost (specialized resources, the default for most cases)
2. Comparative Advantage and Gains from Trade★★☆☆☆⏱ 5 min
Trade allows all parties to consume beyond their individual PPCs, even if one party has an absolute advantage in producing all goods. Two key concepts determine who should specialize to gain from trade:
**Absolute advantage**: The ability to produce more of a good than another entity with the same amount of resources
**Comparative advantage**: The ability to produce a good at a lower per-unit opportunity cost than another entity
The core rule for mutual gains from trade: All parties benefit if they specialize in the good where they have comparative advantage, then trade at a price that falls between the two parties' opportunity costs. For AP exam questions, always calculate per-unit opportunity costs first before assigning comparative advantage.
3. Marginal Analysis for Optimal Decisions★★☆☆☆⏱ 4 min
Marginal analysis is the core decision-making rule for all of microeconomics, used for everything from consumer purchases to firm profit maximization. The term 'marginal' refers to the change in total value from producing or consuming one additional unit of an activity:
**Marginal Benefit (MB)**: The additional total benefit gained from one extra unit of activity
**Marginal Cost (MC)**: The additional total cost incurred from one extra unit of activity
The optimal decision rule: Continue an activity until $MB = MC$, and only proceed with the next unit if $MB \geq MC$. If you are given total benefit or total cost, always calculate marginal values first using the formula:
4. Sunk Costs and Rational Decision-Making★★☆☆☆⏱ 4 min
A sunk cost is any cost that has already been incurred and cannot be recovered, regardless of what decision you make next. The rational decision rule is to ignore sunk costs when making future choices. Including sunk costs in your analysis leads to the sunk cost fallacy, a very common AP exam multiple-choice trap.
Common Pitfalls
Why: Students confuse total lost value with the relevant trade-off for the decision
Why: Students assume the entity that produces more of a good should always produce it
Why: Students forget marginal refers to the change for each additional unit
Why: Students feel they need to 'get their money's worth' from past spending
Why: Students learn constant opportunity cost first and forget that specialized resources are the default in most cases