Basic Economic Concepts

What this unit covers

The topics below follow the published Micro course framework for Unit 1. This unit is worth 12–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Scarcity, Trade-offs & Marginal Analysis13 min · 3 objectivesExplain how scarcity forces trade-offs and creates opportunity cost · Apply marginal analysis by comparing marginal benefit and marginal cost · Identify the utility-maximizing choice using diminishing marginal utility
• The Production Possibilities Curve14 min · 3 objectivesInterpret the PPC as a model of scarcity, trade-offs, and efficiency · Calculate opportunity cost from points on a PPC · Explain why the PPC bows outward and how it shifts with growth
• Comparative Advantage & the Gains from Trade14 min · 3 objectivesDistinguish absolute advantage from comparative advantage · Determine comparative advantage from output or input data · Identify mutually beneficial terms of trade
• Economic Systems & the Circular Flow13 min · 3 objectivesIdentify the three questions every economic system answers · Explain how prices allocate resources without central direction · Trace flows through the product and resource markets
• Terms of Trade & the Output–Input Trap14 min · 3 objectivesCompute opportunity cost correctly from both output and input tables · Determine the range of mutually beneficial terms of trade · Explain why absolute advantage does not determine the gains from trade
• Cost–Benefit Analysis & Sunk Costs13 min · 3 objectivesApply the marginal decision rule to a table of marginal benefits and costs · Explain why sunk costs must be excluded from a forward-looking decision · Distinguish marginal from average and total quantities
• Consumer Choice & Utility Maximization13 min · 3 objectivesApply the utility-maximizing rule equalizing marginal utility per dollar · Explain how diminishing marginal utility generates a downward-sloping demand curve · Separate the substitution effect from the income effect
• Reading the PPC: Shapes, Shifts & Efficiency13 min · 3 objectivesExplain why the PPC is typically bowed outward rather than straight · Classify points as efficient, inefficient or unattainable · Distinguish a movement to the curve from a shift of the curve

Formulas in Unit 1

Every term in Unit 1

All 43 terms we publish for Basic Economic Concepts, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Utility maximization rule

Allocate spending so that MUx / Px = MUy / Py for all goods, subject to the budget constraint. Compare marginal utility per dollar, not raw marginal utility or raw price. Diminishing marginal utility guarantees that reallocating toward the higher-ratio good eventually equalizes the ratios.

Scarcity

Wants exceed the resources available to satisfy them. The reason every economic question is a question about trade-offs.

Opportunity cost

The value of the next-best alternative given up. Not the money spent — the thing forgone.

Marginal analysis

Do one more unit while marginal benefit exceeds marginal cost; stop where MB = MC. Every optimization in this course is a version of this rule.

Marginal benefit

The additional satisfaction or revenue from one more unit. It falls as quantity rises, which is why demand curves slope down.

Why marginal cost rises

The additional cost of producing one more unit. It eventually rises because of diminishing marginal returns, which is why supply curves slope up.

Production possibilities curve (PPC)

All combinations of two goods an economy can produce using its resources fully and efficiently. Inside is inefficient; outside is unattainable.

Why the PPC bows outward

Resources are not equally suited to both goods, so shifting the least-suited resources first is cheap and the best-suited ones later is expensive — increasing opportunity cost.

Constant opportunity cost

A straight-line PPC, meaning resources are perfectly substitutable between the two goods. The slope is the constant trade-off ratio.

Productive efficiency

Producing at the lowest possible cost — any point on the PPC, and in the long run the minimum of ATC.

Allocative efficiency

Producing the combination society most wants, where marginal benefit equals marginal cost. In a market, where P = MC.

Absolute advantage

Producing more output with the same resources. It does not determine who should specialize.

Comparative advantage

Producing at lower opportunity cost. This determines the pattern of specialization and trade.

Calculating opportunity cost from an output table

The opportunity cost of one unit of good A is (units of B) / (units of A) — "other over". The lower value identifies the comparative advantage.

Terms of trade

An exchange ratio both parties accept. It must fall between the two opportunity costs, or one party gains nothing.

Economic vs accounting profit

Accounting profit subtracts explicit costs only. Economic profit subtracts implicit costs too, so it is always the smaller number — and it is the one that drives entry and exit.

Explicit and implicit costs in economic profit

Explicit costs are out-of-pocket payments. Implicit costs are the value of resources the owner already holds — forgone salary, forgone rent on an owned building.

Normal profit

Zero economic profit: revenue exactly covers explicit and implicit costs. The firm is earning what its resources would earn elsewhere, so there is no reason to enter or exit.

Circular flow model

Households supply resources in the factor market and buy goods in the product market; firms do the reverse. Money flows one way, goods and resources the other.

Market economy vs command economy

A market economy allocates through prices and private ownership; a command economy through central direction. The three questions — what, how, and for whom — are answered differently.

Positive vs normative statements

Positive statements can be tested against evidence; normative statements assert what ought to be. "Explain" prompts want positive analysis.

Ceteris paribus

"All else equal." The assumption that makes a two-axis graph legible, and the thing a question relaxes whenever a curve shifts.

Rational self-interest

The behavioral assumption that agents weigh costs and benefits to maximize their own objective. It does not mean selfishness, and it does not require perfect information.

Utility

Satisfaction from consumption. Total utility rises while marginal utility is positive; it peaks exactly where marginal utility hits zero.

Law of diminishing marginal utility

Each additional unit consumed yields less added satisfaction than the last. The underlying reason a demand curve slopes downward.

Utility-maximizing rule

Allocate the budget so MUx / Px = MUy / Py for every good. If MU per dollar is higher for one good, shift spending toward it until the ratios equalize.

The three questions

What to produce, how to produce it, for whom. Every system answers them; they differ only in who decides.

What prices do

Signal relative scarcity and reward whoever responds — both at once. That is what lets a market coordinate without a coordinator.

Product vs resource market

Product market: households pay firms for goods. Resource market: firms pay households for labor, land and capital. Unit 5 is the second one.

Capital is not money

Capital means produced means of production — tools, machines, buildings. Cash, loans and shares are financial assets, not factors of production.

Productive vs allocative efficiency

Productive: on the PPC, nothing wasted. Allocative: at the point society values most, where P = MC. Achieving the first says nothing about the second.

Output vs input tables

Output table: opportunity cost = other ÷ own. Input table (hours per unit): own ÷ other. The ratios INVERT, and this is tested every year.

Absolute vs comparative advantage

Absolute: producing more with the same resources. Comparative: lower opportunity cost. Only the second determines who specializes.

Why nobody has comparative advantage in both

It compares ratios: if one ratio is lower the other must be higher. So each party has one somewhere and trade can benefit both.

Terms of trade range

Beneficial terms lie strictly between the two opportunity costs. At exactly one endpoint, that party is indifferent and gains nothing.

The marginal decision rule

Continue while MB > MC, stop at MB = MC. This becomes MR = MC, MRP = wage, and MSB = MSC — one rule with four sets of labels.

Why a sunk cost cannot distinguish options

Already incurred, unrecoverable, and identical under every option — so it cannot distinguish them. In Unit 3 this becomes the shut-down rule.

Marginal vs average vs total

Marginal is the change from one more unit. Average is total over quantity. Confusing them gives a wrong answer that looks carefully computed.

Diminishing marginal utility

Each extra unit adds less satisfaction, which is why demand slopes down. Total utility keeps rising while marginal utility is positive.

Substitution vs income effect

A price cut makes the good relatively cheaper (substitution) and frees purchasing power (income). Both raise quantity for a normal good.

Inside, on and outside the PPC

Inside: attainable but inefficient — unemployed or misallocated resources. On: productively efficient. Outside: unattainable.

Efficiency gain vs growth

Moving from inside the PPC to the curve is an efficiency gain. Moving the curve outward is growth. Unemployment puts you inside, not inward with it.

Asymmetric PPC shift

Technology improving only good X extends the X intercept and leaves the Y intercept unchanged — the curve pivots rather than shifting uniformly.

What examiners penalize here

• For utility-maximization problems, always convert to **marginal utility per dollar** (MU ÷ price) before comparing goods — never compare raw marginal utilities when prices differ. Buy from whichever good has the higher MU-per-dollar until they equalize and the budget is spent.
• Label PPC diagrams fully: both goods on the axes, and be explicit about whether a change is a **movement along** (reallocation) or a **shift/rotation** (capacity change). Biased technology rotates the curve along one axis — a favorite AP variation.
• On comparative-advantage free-response items, always (1) compute each party’s opportunity costs, (2) assign each good to the lower-cost producer, and (3) give a specific trade rate *between* the two costs. Showing the numbers, not just naming the winner, earns the points.
• When a question asks whether an outcome is "efficient", check which efficiency it means. Productive and allocative efficiency are different conditions, and market-failure questions almost always turn on the second.
• Write both opportunity costs in a labeled two-row table before answering, with units ("1 table = 2 chairs"). The calculation is scored separately from the conclusion, so a correct table earns credit even if you then name the wrong party.
• When a table of totals appears, compute the marginal column in the margin before reading the question. Nearly every quantitative decision item is answered from it.
• Compute the whole utility-per-dollar column first, then allocate the budget in descending order. Working good by good is slower and makes it easy to miss that the budget is exhausted.
• Label both axes with the specific goods and mark the point the question describes before reasoning. Most PPC errors come from answering about the wrong axis rather than from misunderstanding the concept.

Practice Micro

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 6 units of AP Microeconomics

1. Unit 1 · Basic Economic Concepts
2. Unit 2 · Supply & Demand
3. Unit 3 · Production, Cost & Perfect Competition
4. Unit 4 · Imperfect Competition
5. Unit 5 · Factor Markets
6. Unit 6 · Market Failure & Government