Basic Economic Concepts (Macro)

AP Macroeconomics · College Board Unit 1

What this unit covers

• Scarcity, opportunity cost and the factors of production.
• The production possibilities curve.
• Comparative advantage and the gains from trade.
• Demand, supply and market equilibrium as the foundation for macro models.
• Marginal analysis.

At 5–10% of the multiple-choice section this is the lightest unit by weighting, but it is badly underrated. The comparative-advantage calculation and the supply-and-demand model reappear throughout the course, the foreign exchange market and the loanable funds market in Units 4 and 6 are both ordinary supply-and-demand diagrams with unusual axis labels.

Scarcity and opportunity cost

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Resources are scarce while wants are unlimited, so every choice has an opportunity cost, the value of the next best alternative forgone.

Scarcity forces every society to answer three questions: what to produce, how to produce it, and for whom. Different economic systems answer them differently, markets through prices, command economies through central planning, and mixed economies through both, but no system escapes having to answer them.

The factors of production are the scarce resources themselves:

The payments column matters: it is the income side of the circular flow in Unit 2, and it is why total output equals total income.

AP readers expect opportunity cost stated as a specific forgone alternative, not as a sum of money. "The opportunity cost is 5 units of capital goods" earns the point; "the opportunity cost is \$500" usually does not.

The production possibilities curve

The PPC shows the maximum combinations of two goods obtainable with current resources and technology.

• Bowed out (concave) → increasing opportunity cost, because resources are not equally suited to both goods. As you push production of one good further; you must draw in resources progressively less suited to it, so each extra unit costs more of the other good.
• Straight line → constant opportunity cost, meaning resources are equally suited to both.
• Outward shift → economic growth from more or better resources, or improved technology.

A shift can also be lopsided: a technological improvement affecting only one good pivots the curve outward along that axis alone. Reading which axis a shift favours is a common multiple-choice task.

Efficiency has two senses worth separating. Productive efficiency means being on the curve, no output is being wasted. Allocative efficiency means being at the particular point on the curve that society most values. Every point on the frontier is productively efficient; only one is allocatively efficient.

The macro application that matters most: a PPC drawn with capital goods on one axis and consumer goods on the other illustrates the growth trade-off directly. Choosing more capital goods today means fewer consumer goods now, but a larger outward shift later, because capital is itself a factor of production. This is the clearest link between Unit 1 and the long-run growth material in Unit 5.

An economy operating inside the curve is in recession with unemployed resources. Returning to the curve is a recovery, not growth in capacity, AP distinguishes these carefully, and the corresponding AD–AS picture is a rightward AD shift along an upward-sloping SRAS, not an LRAS shift.

Comparative advantage and gains from trade

• Absolute advantage: producing more with the same resources.
• Comparative advantage: producing at a lower opportunity cost.

Comparative advantage determines the pattern of trade, not absolute advantage. A country with an absolute advantage in both goods still gains by specialising, because it cannot have a comparative advantage in both, the opportunity costs are reciprocals, so whichever good one country is relatively better at, the other is relatively better at the remaining one.

The figure is the whole argument in one picture. The US can produce more of both goods, so it has an absolute advantage in both. But its frontier is much flatter: giving up one shoe releases enough resources for 4 refrigerators, whereas in Mexico one shoe costs only 1.25 refrigerators. Mexico is therefore the lower-opportunity-cost producer of shoes, and the US of refrigerators. The point drawn just outside each frontier is what each country can consume after specialising and trading, a combination neither could reach alone.

Calculating it

Put the good you want the cost of on the bottom of the fraction:

Opportunity cost of 1 unit of X = (units of Y given up) ÷ (units of X gained)

Watch which kind of problem you have been given. AP sets two, and they invert the arithmetic:

• Output problems give the quantity each country can produce with fixed resources ("with 40 workers, the US can make 10,000 shoes"). Divide the other good by your good.
• Input problems give the resources needed per unit of output ("it takes 3 hours to make a shoe"). Here the country needing fewer inputs has the absolute advantage, and the opportunity cost ratio flips relative to the output case.

A quick check that catches most errors: the country with the comparative advantage in a good must be the one that gives up less of the other good to make it.

Terms of trade must lie between the two countries' opportunity costs for both to gain. Outside that range, one country would do better producing the good itself.

Worked example

Using the same resources, Ando can produce 20 shirts or 10 radios; Bell can produce 30 shirts or 30 radios.

Ando: 20 shirts ÷ 10 radios = 2 shirts per radio
Bell: 30 shirts ÷ 30 radios = 1 shirt per radio

Bell has an absolute advantage in both. But Bell gives up only 1 shirt per radio while Ando gives up 2, so Bell has the comparative advantage in radios and Ando in shirts.

Any rate between 1 and 2 shirts per radio benefits both. At 1.5, Bell receives more than its 1-shirt cost and Ando pays less than its 2-shirt cost.

Check it the other way round, since the exam may ask for either good:

Ando: 10 radios ÷ 20 shirts = 0.5 radios per shirt
Bell: 30 radios ÷ 30 shirts = 1 radio per shirt

Ando gives up fewer radios per shirt, confirming Ando's comparative advantage in shirts. The two calculations must agree; if they do not, one of the fractions is upside down.

Demand, supply and equilibrium

The micro model underlies the macro one, and AP Macro tests it in its own right, particularly in the foreign exchange and loanable funds markets later in the course.

• Demand slopes down (substitution and income effects); supply slopes up (rising marginal cost).
• Equilibrium is where quantity demanded equals quantity supplied; above it a surplus, below it a shortage, both correcting through price.
• A change in the good's own price causes a movement along; anything else shifts the curve.

Shifters of demand: income (and whether the good is normal or inferior), the prices of substitutes and complements, tastes, expectations, and the number of buyers.

Shifters of supply: input prices, technology, taxes and subsidies, expectations, and the number of sellers.

When both curves shift, one outcome is determinate and the other ambiguous, exactly the pattern seen with AD and SRAS in Unit 3:

Saying which is ambiguous, and why, is the expected answer rather than a guess.

Marginal analysis

Rational decision-makers continue an activity while marginal benefit exceeds marginal cost and stop where MB = MC. The same rule reappears throughout AP Macro, in the money market, the loanable funds market and fiscal policy decisions.

Sunk costs are irrelevant to marginal decisions: money already spent cannot be recovered, so it cannot be affected by what you choose now. Only costs that change with the decision belong in the calculation.

Worked example

An economy produces only capital and consumer goods and is currently at a point inside its PPC, with unemployment at 9%.

Idle resources mean output can rise without any sacrifice of the other good → moving to the curve raises production of both → this is a recovery, shown on AD–AS as AD shifting right along SRAS.

If instead the economy is on the curve and chooses to produce more capital goods; it must give up consumer goods now, a genuine opportunity cost, but the additional capital shifts the PPC outward in future periods. That is the trade-off between present consumption and future growth.

Second worked example: marginal analysis with numbers

A firm is deciding how many units to produce. Marginal benefit falls and marginal cost rises as output increases:

The optimum is 3 units. Producing the fourth would add \$20 of benefit at \$40 of cost, destroying \$20 of value.

Note what the rule does not say: it does not say maximise marginal benefit, or produce while benefit is positive. It says produce while the benefit of the next unit covers its cost. If the firm had already spent \$100 on equipment; that would be sunk and would not change the answer at all.

Common exam mistakes

• Calling a point outside the PPC "inefficient"; it is unattainable.
• Assigning comparative advantage to the country with the larger output.
• Giving terms of trade outside the range between the two opportunity costs.
• Inverting the opportunity cost fraction, or treating an input problem as an output problem.
• Shifting the PPC outward to represent a recovery from recession.
• Stating opportunity cost in dollars rather than as forgone goods.
• Treating a bowed-out PPC as showing constant opportunity cost.
• Confusing productive efficiency (on the curve) with allocative efficiency (the right point on it).
• Including sunk costs in a marginal decision.

Exam technique

Label PPC axes with the two goods by name. For the growth trade-off. Use capital goods and consumer goods, since that is the version AP builds on later.

Always show the division when computing opportunity cost, method earns credit independently of the conclusion, and it lets you keep marks when the arithmetic slips. Then sanity-check by asking which country gives up less.

Decide whether the scenario describes using idle resources (move towards the curve) or acquiring more resources (shift the curve). Choosing the wrong one costs the diagram points regardless of what the prose says.

When a question involves two goods and two countries, build the opportunity-cost table before writing anything. Four numbers, then the comparison, the answer falls out, and the table itself is usually creditworthy.

Quick revision

• Opportunity cost = the next best alternative forgone, stated in goods.
• Factors: land, labour, capital, entrepreneurship, earning rent, wages, interest, profit.
• On the PPC = efficient; inside = inefficient; outside = unattainable.
• Bowed out = increasing opportunity cost; straight line = constant.
• Productive efficiency = on the curve; allocative efficiency = the best point on it.
• Capital goods today → larger outward PPC shift tomorrow.
• Comparative advantage = lower opportunity cost; it drives trade, not absolute advantage.
• Check output versus input problems before dividing.
• Terms of trade lie between the two opportunity costs.
• Own price → movement along; anything else → shift. Both curves shifting → one variable indeterminate.
• Optimise where MB = MC; ignore sunk costs.