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AP® Microeconomics
All six College Board units, the four skill categories, and the graphs: built for the 60 MC and the 3 FRQs.
Start Unit 1 free. Unit 1: Basic Economic Concepts is open to everyone, no account needed. Other topics are locked.
Unit 1: Basic Economic Concepts
THE BIG PICTURE. Unit 1 (12–15%) lays the foundational logic of how rational agents make decisions under scarcity. Four core ideas dominate: scarcity (why economics exists), opportunity cost (the true cost of any choice), comparative advantage (why trade benefits both parties), and marginal analysis (the universal decision rule). These tools recur in every later unit.
SCARCITY AND RESOURCE ALLOCATION
Resources are limited; wants are unlimited. Therefore choices must be made, and every choice has a cost. Three fundamental questions every economy must answer:
- What to produce?
- How to produce it?
- For whom to produce it?
Different economic systems answer these differently:
- Market economy: decentralized; consumers and firms decide via prices.
- Command economy: central planners decide.
- Mixed economy: most real-world systems; combines market with government intervention.
Factors of production (the resources used to make goods/services)
- Land (natural resources). Payment: rent.
- Labor (human work). Payment: wages.
- Capital (machines, buildings, tools; NOT financial capital). Payment: interest.
- Entrepreneurship (organizing the other three, bearing risk). Payment: profit.
THE PRODUCTION POSSIBILITIES CURVE (PPC)
A graph showing the maximum combinations of two goods an economy can produce using all available resources efficiently. Axes: the two goods.
Reading the PPC:
- On the curve: efficient (all resources used at full capacity).
- Inside: inefficient (unused or misallocated resources, e.g. unemployment).
- Outside: unattainable with current resources (requires growth).
- Bowed-out (concave): increasing opportunity cost (resources not equally productive for both goods). Most realistic case.
- Straight-line: constant opportunity cost (resources equally well-suited).
Shifts of the PPC:
- Outward: economic growth (more resources, better technology, more workers).
- Inward: loss of resources (disaster, war, population decline).
- Asymmetric: improvement in technology for ONE good shifts only that axis outward.
Constant versus increasing opportunity cost A straight PPC means every unit of wheat costs the same amount of cloth. A bowed-out PPC means each extra unit costs more: here the first 20 wheat cost 3.4 cloth but the last 20 cost 44.7.
Shifting the PPC More resources or better technology for both goods shift the whole curve out. A change that helps only one good pivots the curve out along that good's axis while the other intercept stays put.
WORKED EXAMPLE: OPPORTUNITY COST FROM A PPC TABLE
A country produces only robots and wheat. Its production possibilities are:
| Point | Robots | Wheat (tons) |
|---|---|---|
| A | 0 | 100 |
| B | 10 | 90 |
| C | 20 | 70 |
| D | 30 | 40 |
| E | 40 | 0 |
- Step 1, cost of each move. A to B: 10 robots cost 10 tons of wheat (1 ton per robot). B to C: 20 tons (2 per robot). C to D: 30 tons (3 per robot). D to E: 40 tons (4 per robot).
- Step 2, name the pattern. Each extra robot costs more wheat (1, 2, 3, then 4 tons), so this PPC shows increasing opportunity cost and is bowed out. Farmland and farm workers that are poorly suited to robot-making get moved last.
- Step 3, classify points. 20 robots and 60 tons is inside the curve (point C shows 70 tons is possible), so it is inefficient. 30 robots and 50 tons is unattainable (point D shows only 40 tons is possible with 30 robots).
If every move had cost the same 2.5 tons per robot, the PPC would be a straight line: constant opportunity cost.
PRACTICE: MOVE ALONG, MOVE INSIDE, OR SHIFT?
| Event | What happens on the PPC graph |
|---|---|
| Unemployment rises in a recession | The point moves inside the PPC; the curve does not shift |
| A new fertilizer raises wheat yields only | The PPC pivots outward along the wheat axis only |
| An earthquake destroys factories and farms | The whole PPC shifts inward |
| Immigration adds workers | The whole PPC shifts outward |
| The country decides to build more robots | A movement along the PPC; the cost is wheat given up |
| The country makes more capital goods today | A movement along today's PPC, and a larger outward shift in the future |
CASE STUDY: DETROIT GOES TO WAR (1942)
After the United States entered World War II, automakers halted civilian car production in early 1942; Ford's civilian car and truck lines stopped on February 10, 1942. No new civilian cars were built for the rest of the war. Auto plants turned out tanks, aircraft engines, trucks, and weapons instead, and civilians faced rationing of tires and gasoline.
- Concept: trade-offs and opportunity cost. Military output came at the cost of consumer goods.
- Graph: a movement along a PPC with consumer goods on one axis and military goods on the other, not a shift of the curve.
- Lesson: the real cost of the war production was the civilian goods the economy gave up.
OPPORTUNITY COST
The value of the next-best alternative given up when a choice is made. NOT the same as monetary price; includes time, foregone benefits, and any other loss.
On the PPC: opportunity cost of producing more of Good X = the units of Good Y given up (slope of PPC at that point).
Example: if a country can produce either 10 cars or 20 computers, the opportunity cost of 1 car = 2 computers; the OC of 1 computer = 0.5 cars.
COMPARATIVE ADVANTAGE AND GAINS FROM TRADE
- Absolute advantage: producer that can make MORE of a good with the same resources (or the same amount with fewer resources).
- Comparative advantage: producer with the LOWER OPPORTUNITY COST.
Trade is mutually beneficial when each party specializes in the good for which it has comparative advantage. Even a country with absolute advantage in BOTH goods gains by specializing in the one where its comparative advantage is greatest.
Terms of trade: the ratio at which goods are exchanged. Must fall between the two opportunity costs for both parties to gain.
Example: Country A: 1 car costs 2 computers. Country B: 1 car costs 4 computers. A has CA in cars (lower OC); B has CA in computers. Trade ratio anywhere between 2 and 4 computers per car benefits both.
WORKED EXAMPLE: COMPARATIVE ADVANTAGE FROM AN INPUT TABLE
Some questions give the time needed to make one unit instead of total output. Pat and Lee each work 12 hours a day:
| Worker | Hours to bake 1 cake | Hours to sew 1 shirt |
|---|---|---|
| Pat | 2 | 4 |
| Lee | 3 | 3 |
- Absolute advantage: Pat bakes faster (2 hours versus 3), so Pat has the absolute advantage in cakes. Lee sews faster (3 hours versus 4), so Lee has the absolute advantage in shirts.
- Opportunity cost of 1 cake: Pat's 2 hours could have made 2/4 = 0.5 shirt; Lee's 3 hours could have made 3/3 = 1 shirt.
- Opportunity cost of 1 shirt: 4/2 = 2 cakes for Pat; 3/3 = 1 cake for Lee.
- Comparative advantage: Pat in cakes (0.5 shirt is lower), Lee in shirts (1 cake is lower).
- Mutually beneficial terms of trade: between 0.5 and 1 shirt per cake, for example 3 cakes for 2 shirts.
Shortcut: with an input table (hours per unit), the opportunity cost of X is (hours for X) ÷ (hours for Y). With an output table (units per day), it is flipped: (units of Y) ÷ (units of X). Mixing up the two is the most common Unit 1 error.
COST-BENEFIT ANALYSIS
Rational decision-makers compare the benefits and costs of a choice. They choose the action when benefits exceed costs.
Key distinction:
- Total benefit/cost: the full value/cost of an entire activity.
- Marginal benefit (MB): additional benefit from ONE MORE unit.
- Marginal cost (MC): additional cost of ONE MORE unit.
Decision rule: keep producing or consuming as long as MB ≥ MC. Stop where MB = MC. This is the profit-maximizing / utility-maximizing rule that drives most micro decision-making.
Sunk costs are costs already paid and irrecoverable. Rational decision-makers IGNORE sunk costs and focus only on marginal (forward-looking) costs.
WORKED EXAMPLE: FINDING THE OPTIMAL QUANTITY WITH MB AND MC
A student is deciding how many hours to spend at a tutoring center. The center charged a one-time $20 sign-up fee (already paid) and charges $15 per hour. The student values the hours as follows:
| Hour | Total benefit | Marginal benefit | Marginal cost |
|---|---|---|---|
| 1 | $40 | $40 | $15 |
| 2 | $70 | $30 | $15 |
| 3 | $90 | $20 | $15 |
| 4 | $105 | $15 | $15 |
| 5 | $115 | $10 | $15 |
- Marginal benefit is the change in total benefit: 70 − 40 = $30 for the 2nd hour, and so on.
- Decision rule: take each hour whose MB is at least its MC. Hours 1 to 4 qualify; the 5th hour adds $10 of benefit for $15 of cost. Optimal choice: 4 hours, where MB = MC.
- Check with totals: net benefit (total benefit minus hourly charges) is $45 at 3 hours (90 − 45), $45 at 4 hours (105 − 60), and only $40 at 5 hours (115 − 75). The 4th hour adds exactly as much as it costs, which is why MB = MC marks the top.
- The $20 fee is sunk. It was paid whether the student studies 0 hours or 5, so it changes none of the comparisons.
CASE STUDY: THE CONCORDE FALLACY
Concorde, the supersonic airliner developed jointly by Britain and France, carried passengers from 1976 until 2003. Only 20 were built and 14 entered airline service, and the program is widely described as one whose backers kept investing to justify what had already been spent. Biologists Richard Dawkins and Tamsin Carlisle, writing in Nature in 1976, coined the name "Concorde fallacy" for this error, and psychologists now use it as another name for the sunk cost effect.
- Concept: sunk costs cannot be recovered, so they should not affect whether to continue.
- Rule: continue only if future benefits exceed future costs (marginal benefit at least marginal cost from here on).
- Lesson: "we have already spent too much to stop" is not an economic argument for spending more.
MARGINAL ANALYSIS AND CONSUMER CHOICE
Utility: satisfaction or happiness from consumption. Marginal utility (MU): additional utility from one more unit consumed. Law of diminishing marginal utility: as you consume more of a good, the additional utility from each successive unit DECREASES. (First slice of pizza is great; tenth slice is unpleasant.)
Utility-maximizing rule (for choosing between goods with prices and )
This is the rule for getting the most "utility per dollar." A rational consumer allocates spending so that the marginal utility per dollar is EQUAL across all goods.
Example: if a slice of pizza (price $2) gives , and a soda (price $1) gives : pizza yields 10 utils per dollar, soda yields 8. Buy more pizza. Continue until .
PRACTICE: WHERE SHOULD THE NEXT DOLLAR GO?
| Situation | Utils per dollar | Best move |
|---|---|---|
| Tacos: MU 30, price $3. Burritos: MU 40, price $5 | 10 versus 8 | Buy more tacos |
| Songs: MU 6, price $1. Movies: MU 60, price $12 | 6 versus 5 | Buy more songs |
| Coffee: MU 12, price $4. Tea: MU 9, price $3 | 3 versus 3 | Already optimal if the budget is spent |
| The price of good A rises; MU unchanged | falls | Buy less A: the law of demand |
Scope note: indifference curves are excluded from the CED; comparing marginal utility per dollar is what the exam tests.
THE CIRCULAR FLOW MODEL
A simple model showing how money and goods flow between households and firms:
- Households supply factors (labor, land, capital) in the factor market → firms pay wages, rent, interest.
- Firms produce goods/services in the product market → households spend income to buy them.
Money flows clockwise; goods/services flow counterclockwise. The two streams are equal in value: total spending = total income = total output (the bookkeeping identity behind GDP).
Scope note: the AP Microeconomics CED does not list the circular flow model (it belongs to AP Macroeconomics), so treat this section as background rather than exam content.
EXAM CONNECTIONS. Unit 1 problems appear as quick MC: identify the comparative-advantage country, read OC from a PPC, apply MB = MC to find optimal quantity. The PPC is a standard FRQ graphing skill: bowed-out shape, labeled axes, point inside/on/outside marked. Marginal analysis recurs in EVERY later unit (firm output, factor hiring, externality correction).
Key Terms
Scarcity
The condition that resources are insufficient to satisfy all wants. The fundamental economic problem.
Factors of Production
Land (natural resources; payment = rent), labor (work; wages), capital (machines/tools; interest), entrepreneurship (organization/risk; profit). The four inputs to production.
Opportunity Cost
Value of the next-best alternative given up. Not just money: includes time, foregone benefits, any other loss.
Production Possibilities Curve (PPC)
Graph of maximum two-good combinations using all resources efficiently. Bowed-out = increasing OC; straight = constant OC.
Absolute Advantage
Ability to produce MORE of a good than another producer using the same resources. NOT the basis for trade.
Comparative Advantage
Ability to produce a good at LOWER OPPORTUNITY COST than another producer. THE basis for mutually beneficial trade.
Terms of Trade
Ratio at which two goods are exchanged. Must lie between the two trading partners' opportunity costs for both to gain.
Marginal Benefit (MB)
Additional benefit from one more unit of consumption or production. Usually decreasing (diminishing returns).
Marginal Cost (MC)
Additional cost of producing or consuming one more unit. Usually increasing (rising input costs / opportunity).
Marginal Decision Rule
Keep doing the action while MB ≥ MC; stop where MB = MC. The universal optimization principle in microeconomics.
Sunk Cost
Cost already paid, irrecoverable. Rational decision-makers ignore sunk costs and look only forward at marginal costs.
Utility
Satisfaction from consumption, measured in "utils" (an abstract unit). Cardinal in textbooks; ordinal in modern theory.
Diminishing Marginal Utility
As more of a good is consumed, the additional utility from each unit FALLS. Universal pattern, foundation of downward-sloping demand.
Utility-Maximizing Rule
across all goods. Equalize marginal utility per dollar. Yields the consumer's optimal mix.
Circular Flow Model
Money flows clockwise (firms pay wages → households spend); goods flow counterclockwise (households supply labor → firms supply goods).
Exam Tips
- Marginal decision rule (MB = MC) is universal. Use it for: firm output (Unit 3), factor hiring (Unit 5), correcting externalities (Unit 6). This single rule unifies micro.
- Ignore sunk costs. Only marginal/forward-looking costs matter. The sunk-cost fallacy is a common MC trap.
- Comparative advantage uses opportunity cost, not absolute output. Compute OC = alternative good / target good, then compare.
- For PPC graphing: bowed-out shape, axes labeled with the two goods, point on (efficient) / inside (inefficient) / outside (unattainable) clearly marked.
- Utility-maximizing rule: equal across all goods. To maximize utility per dollar.
- Factor of production payments: land/rent, labor/wages, capital/interest, entrepreneurship/profit. Memorize the pairing.
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