Every later chapter is one of these five wearing a different hat — supply and demand is trade-offs with a graph, elasticity is marginal thinking with a ratio. Read a chunk, then answer the checkpoint from memory before scrolling on.
One idea and two vocabulary splits that show up on every exam in the course.
Economics is the study of how people allocate scarce resources. That is the whole subject. The engine underneath it is scarcity.
Scarcity does not mean "rare." Water is abundant and still scarce — you cannot have unlimited amounts at zero cost. A signed photo of a band nobody likes is rare and not scarce, because nobody wants it. Rare is about supply; scarce is about supply relative to wants.
So every society answers the same three questions: what to produce, how, and who gets it. The rest of the course is machinery for answering those.
| Split | One side | Other side | How to tell |
|---|---|---|---|
| Scale | Micro — individual decision-makers: a household, a firm, one market | Macro — the economy as a whole: inflation, unemployment, growth | Count the actors. One market or one chooser → micro. |
| Claim type | Positive — testable, factual: "a $15 minimum wage reduces teen employment" | Normative — value judgement: "a $15 minimum wage is fair" | Could data settle it? Yes → positive. Words like should, fair, ought → normative. |
Positive statements can be wrong and still be positive — "the earth is flat" is a positive statement. The test is whether evidence could decide it, not whether the claim is true.
People respond to incentives, predictably, and often in ways the person designing the incentive did not plan. This foundation is where most exam questions about "unintended consequences" come from.
An incentive is anything that changes the costs or benefits of an action. Two classifications matter, and exam questions almost always turn on the second one.
Mandatory seat belts have a direct effect: a given crash is more survivable, so deaths per crash fall. They also have an indirect effect: feeling safer, some drivers drive a little faster and follow a little closer, so the number of crashes rises — including crashes involving pedestrians and cyclists, who got no seat belt. The net effect on total deaths is an empirical question, and that is exactly the point. Identifying the indirect incentive is the skill being tested; you are not expected to know the net sign from memory.
Try it on a real policy. The model below is the actual arithmetic of a city bag fee — how many shoppers switch, how much plastic that saves, and how much of the saving the indirect incentive eats back.
Scarcity forces choice, and choice means giving something up. It gets its own foundation because students collapse it into opportunity cost, and the exam distinguishes them.
There is no such thing as a free lunch. Every choice costs you the alternatives — including the ones that feel free. A free concert costs you the three hours.
A trade-off is the fact that choosing one thing means giving up others. It is plural and qualitative: "studying tonight means no gym, no party, no shift at work."
An opportunity cost is the single highest-valued alternative you gave up, and it carries a number. "Studying tonight cost me the $80 shift" — the one you'd actually have picked.
Trade-offs are the menu of things forgone. Opportunity cost is the best item on that menu. If a question says "list the trade-offs" and you name one thing, you lost points; if it says "what is the opportunity cost" and you name three, you also lost points.
Trade-offs are why the production possibilities curve in Chapter 3 slopes downward and why budget constraints in Chapter 4 are straight lines. When you meet those, you are meeting this foundation with axes attached.
The most tested idea in the chapter. Three rules cover every version of it you will see.
Opportunity cost is the value of the highest-valued alternative you gave up. Three rules follow, and each one is a graded exam trap.
Explicit costs are payments you actually make: tuition, rent on a shop, wages to employees. Implicit costs are the value of resources you already own and use up: the salary you did not earn, the rent you did not collect on your own building, the interest your own capital did not make.
Accountants count explicit costs. Economists count both. That is the entire difference between accounting profit (revenue − explicit) and economic profit (revenue − explicit − implicit), and it is why a business can be "profitable" and still worth shutting down.
If a cost happens whether or not you choose the thing, it is not part of the opportunity cost of choosing it. Food and housing while you are in college are the classic case: you eat either way, so ordinary living expenses are not a cost of attending. Only the difference counts — if dorm housing costs $4,000 more than living at home would, that $4,000 is in.
A sunk cost is already spent and unrecoverable. It cannot be an opportunity cost of a future decision, because no future choice can get it back. You paid $60 for a concert ticket, you now feel ill: the $60 is gone in both worlds. The only live question is whether a miserable evening out beats a comfortable evening in.
Here is Rule 1 and Rule 2 as arithmetic. Toggle the living-expenses switch to watch the most common wrong answer inflate the number.
Decisions are rarely all-or-nothing. They are "a bit more or a bit less," and the rule for finding the right amount is the most reusable tool in the course.
Marginal means "one more unit." Marginal benefit is the extra benefit from one more unit; marginal cost is the extra cost. Thinking at the margin means asking "is the next one worth it?" rather than "is this activity worth doing at all?"
Keep going while MB > MC. Stop where MB = MC. That point maximises net benefit — total benefit minus total cost — and it is the same rule that will later give you profit maximisation, the optimal consumption bundle, and the efficient quantity in a market.
The model below computes it unit by unit. Drag the quantity and watch net benefit — the bottom curve — peak exactly where the two lines cross, and crash if you follow either wrong rule.
The least intuitive foundation, because ordinary language treats deals as win–lose. They aren't.
Voluntary trade creates value. Nothing new is manufactured in the moment of exchange, yet both sides end up better off. That is not a paradox — value is not a property of an object, it is a relationship between an object and a person. Move the object to the person who values it more and total value has gone up.
So trade is positive-sum: both parties gain, or at minimum neither loses, because either party could have walked away. This is why "if I gained, they must have lost" is wrong, and why it is wrong for international trade too.
Trade is worth doing when people value things differently — usually because they can produce things at different opportunity costs. That is comparative advantage, and it is the whole of Chapter 2. Keep the shape of it in mind now: each side specialises where its own opportunity cost is lowest, then trades for the rest.
Drag the price below. Watch the two surplus bars trade size against each other while their sum never moves — that is the one result in this applet worth memorising.
Shuffled, not blocked — mixing topics beats grouping them (d ≈ 0.35) even though it feels harder. Feeling fluent is not the goal; being right in week 10 is.
Half the value of a model is knowing which numbers to trust. Everything here is computed, not looked up. The honest boundary:
| Model | Genuinely computed | Assumed / simplified |
|---|---|---|
| Bag fee | Share switching from a uniform distribution of hassle costs; bags used; fee revenue; plastic avoided and the bin-liner offset, by mass. | Hassle costs are uniform (real ones aren't). Trips, bags/trip, and bag masses (5.5 g vs 20 g liner) are fixed stand-ins, not survey data. No price effect on groceries. |
| Degree cost | Explicit, implicit and total economic cost; the tuition-only answer; the gap between them; break-even years from the salary gain. | No discounting — a real present-value calculation would shrink later gains. No tuition inflation, no graduation risk, no part-time work. |
| Marginal thinking | MB and MC per unit from linear schedules; total benefit and cost by summation; net benefit at every quantity; the optimum by the MB = MC crossing; the net benefit the overshoot rule gets. | Whole hours only, and both schedules are linear. Real MB and MC curve. |
| Trade surplus | Consumer surplus, producer surplus, total value created, whether a trade occurs, and the bargaining range. | One buyer and one seller. A market with many of each is Chapter 4 — that's where these bars become the triangles under supply and demand. |
Checkpoint items are scheduled, not just listed. A question you miss returns at the very next checkpoint; one you get right returns two checkpoints later; get it right twice and it retires to the chapter quiz. That's deliberately plainer than a real spaced-repetition algorithm, because meta-analysis finds expanding and uniform intervals perform about the same (g = 0.034) — the thing that matters is the number of successful retrievals, not the cleverness of the gaps.
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