Lecture: Scarcity, Opportunity Cost, the PPF & Economic Models

Lecture script · about 15–20 minutes spoken · Week 1

Opening

Picture three people making three very different choices. A nursing student is deciding whether to take an extra shift or spend the night studying. A restaurant owner is deciding whether to open a second room or keep the money in savings. A city council is deciding between a new park and repaired roads. They have nothing in common, except one thing. Each of them has to give something up to get something else.

That one thing is what this course is about, and it's where we start. This is Week 1, so nothing earlier is assumed. You don't need any economics background, and the only math in this lecture is adding, subtracting, multiplying, and dividing.

Here's the plan. First, I'll show you how to put a real price on a choice. Then we'll scale that idea up to a small island that gathers fish and coconuts, draw the first graph of the course, and read what it says about costs. After that, I'll talk about what an economic model leaves out and why. We'll finish by sorting claims into the kind evidence can settle and the kind values decide.

Next week, in Week 2, Comparative Advantage, Specialization & the Gains from Trade, you'll use everything from this lecture to see why two people who trade can both come out ahead.

1. Price a choice: scarcity and opportunity cost

Let's start with you on a free evening. A band you love is playing, and the ticket costs 60 dollars. You were also offered a shift for those same 4 hours, and it pays 15 dollars an hour. You can't do both. You have one evening, and you have more things you'd like to do with it than it can hold.

That squeeze has a name. Scarcity is the condition in which people's wants outrun the time, money, and other resources available to meet them, so every use of a resource means giving up another use. Scarcity isn't the same as being poor. The richest person you can imagine still gets only 24 hours in a day.

Because of scarcity, every choice is a trade-off. And the thing you give up has a value. Opportunity cost is the value of the single best alternative given up to make a choice, counting time and forgone earnings as well as any money paid.

So what does the concert really cost? Start with the money you hand over: 60 dollars. Now add what you give up by not working. That's 4 hours at 15 dollars an hour, which comes to 60 dollars of pay you never earn. Put the two together, and the evening costs you 120 dollars, not 60.

The forgone pay is every bit as real as the ticket. If the concert is worth more than 120 dollars to you, go and have a great time. If it isn't, the shift is the better use of your evening. Either way, you're choosing with the full price in front of you.

Notice the word single in that definition. You might have several other things you could do with the evening: work, study, or sleep. The opportunity cost is the value of the best one, not all of them added up.

Pause and try this. Suppose the shift were already full, so your best alternative was a free night at home. Would the concert's opportunity cost still be 120 dollars?

Take a moment and think about what you'd really be giving up.

Here's the answer. No. With the shift gone, you'd no longer be giving up 60 dollars of pay, so the cost would fall back toward the 60 dollars of the ticket plus whatever that quiet night at home is worth to you. Opportunity cost always depends on what your best alternative really is.

2. Draw the trade-off: the production possibilities frontier

Now let's take that same idea and scale it up from one person to a whole economy. Imagine a small island with a fixed number of workers. Each day, they can gather coconuts from the trees, catch fish in the lagoon, or do some of each. The numbers I'm about to use are made up for this course to make the logic easy to see. They aren't data from any real place.

If everyone gathers coconuts, the island gets 105 coconuts and no fish. If everyone fishes, it gets 60 fish and no coconuts. In between, there's a whole menu. With 10 fish, it can still gather 100 coconuts. With 20 fish, 90 coconuts. With 30 fish, 75. With 40 fish, 55. With 50 fish, 30. And with all 60 fish, zero coconuts.

Economists draw that menu as a graph. The production possibilities frontier is a graph of every combination of two goods an economy can make when it uses all of its resources fully and efficiently; it is often shortened to PPF.

Here's the frontier in words. Put fish per day along the bottom axis, running from 0 to 60. Put coconuts per day up the side axis, running from 0 to 105. The curve starts high on the side axis, at 105 coconuts and no fish. It sweeps down and to the right, through each of those combinations, and lands on the bottom axis at 60 fish and no coconuts.

Here's the key thing about that curve. It's a menu, not a wish list. It tells you what the island can make. It doesn't tell you what the islanders want, and it doesn't pick a combination for them. That choice depends on what they value.

Pause and try this. The island is making 30 fish and 75 coconuts. If it wants 40 fish instead, what has to happen to its coconuts?

Picture the workers walking from the trees down to the lagoon.

Here's the answer. Its coconuts have to fall, from 75 to 55. Moving along the frontier toward more fish always means fewer coconuts, because every worker who starts fishing stops climbing trees.

3. Read the slope: opportunity cost rises along the frontier

Every step along that curve has a price, and the price of more fish is paid in coconuts. That price is the slope of the frontier, and economists read it as an opportunity cost.

Let's compare two steps of 10 fish. The first 10 fish take the island from 105 coconuts down to 100. That's 5 coconuts given up for 10 fish, or half a coconut per fish. Now look at the last 10 fish. They take the island from 30 coconuts down to zero. That's 30 coconuts given up for 10 fish, or 3 coconuts per fish. Same number of fish, and a much higher price.

In fact, if you walk the whole curve in steps of 10 fish, the coconuts given up climb steadily: 5, then 10, then 15, 20, 25, and finally 30. Add those up and you get 105, every coconut the island can gather.

Why does the price keep rising? Here's a short story. When the island first starts fishing, it sends the workers who are great at fishing and only so-so at climbing trees. They catch plenty of fish and leave few coconuts behind. But as the island keeps shifting, it runs out of natural fishers. Eventually it has to send its best tree climbers down to the water. They're clumsy with a net, and they leave a lot of coconuts hanging. So each new fish costs more coconuts than the last.

That rising cost gives the frontier its shape. It bows outward, bulging away from the corner where the two axes meet. Near the top, the curve is fairly flat, because a few coconuts buy a lot of fish. Near the bottom, it's steep, because a lot of coconuts buy only a few fish.

A straight-line frontier would mean every fish costs the same number of coconuts, no matter how many the island catches. That only happens if every worker is equally good at both jobs. It's a special case, not the rule.

Pause and try this. Between 20 and 30 fish, the island gives up 15 coconuts. Between 40 and 50 fish, it gives up 25. Which step has the higher opportunity cost per fish, and why?

Think about who's being moved to the lagoon at each step.

Here's the answer. The step from 40 to 50 fish. It costs 25 coconuts instead of 15 for the same 10 fish, because by then the island is moving workers who are better at climbing trees than at fishing.

4. Test three combinations: on, inside, and outside the frontier

The frontier splits every possible combination into three groups, and you can test any combination by comparing it with what the curve allows at the same number of fish.

Start with 30 fish and 75 coconuts. At 30 fish, the frontier gives exactly 75 coconuts, so this combination sits right on the curve. An efficient point is a combination on the production possibilities frontier, where the economy cannot make more of one good without making less of the other.

Now try 30 fish and 50 coconuts. At 30 fish the island could have 75 coconuts, so this combination leaves 25 coconuts on the trees. It sits inside the frontier. The island can easily reach it, but some workers are idle, or they're doing jobs that don't suit them. Inside means attainable but wasteful.

Last, try 40 fish and 70 coconuts. At 40 fish the frontier gives only 55 coconuts, so this combination asks for 15 more than the island can gather. It sits outside the frontier. With the workers and tools it has now, no plan gets there. Outside means unattainable.

Pause and try this. A friend says that 30 fish and 50 coconuts is impossible because it's inside the frontier. Is your friend right?

Give it a second, and think about what inside really means.

Here's the answer. Your friend has it backwards. The island can make 30 fish and 50 coconuts any day it likes; it only has to waste some effort to do it. It's the combinations outside the frontier, like 40 fish and 70 coconuts, that are out of reach.

5. Grow the island: an outward shift of the frontier

The frontier is drawn for the island's workers, skills, and tools as they are right now. Change those, and the whole curve can move.

Suppose the island gains more workers, builds better fishing nets, or learns a faster way to climb trees. With more or better resources, it can make more of both goods. The whole frontier shifts outward, away from that corner where the axes meet. Some combinations that sit outside the frontier right now, such as 40 fish and 70 coconuts, could come within reach. That outward shift of the whole frontier is how economists picture growth.

There's an important difference hiding here, and I want you to see it coming. Moving along the frontier is one thing. Moving the frontier itself is another. If the island slides along the curve from 30 fish and 75 coconuts to 40 fish and 55 coconuts, it gets 10 more fish, but it gives up 20 coconuts. That's a trade-off. After an outward shift, though, the island can have more fish without giving up any coconuts at all.

Here's an everyday way to see it. Shuffling the hours in your week between work and study is moving along your own frontier. Learning to type much faster is shifting the frontier, because now you can get more of both done.

Pause and try this. The island moves from 30 fish and 75 coconuts to 40 fish and 55 coconuts. Is that growth?

Hold onto the difference between moving along and moving the curve.

Here's the answer. No. The island got 10 more fish only by giving up 20 coconuts, so it slid along the same frontier. Growth would be a shift of the whole curve outward, which lets the island have more of both.

6. Build a model: simplify on purpose, all else equal

You've now used your first economic model, and you may have noticed how much the island leaves out. That's on purpose.

Think about a bus route map. It leaves out every building and tree, and it bends the streets into neat lines. That's exactly why it works. It answers one question, which bus gets you where you're going, without burying you in detail. An economic model is a simplified picture of how part of the economy works, which keeps the few forces that matter for one question and leaves out the rest on purpose. The island answers one question: what does more of one good cost in terms of another?

Models lean on one key assumption. Ceteris paribus is Latin for "all else equal": the assumption that every influence a model leaves out stays constant while one thing changes. When the island moves from 30 fish to 40, ceteris paribus says the number of workers, the weather, and the tools all stay put. Only the split of work changes. Without that assumption, you couldn't tell what caused what.

Economics also has two main branches. Microeconomics is the study of the choices made by individual households and firms and of how single markets work. Macroeconomics is the study of the economy as a whole, including total output, the overall price level, unemployment, and money. This course is microeconomics. Your concert evening and the island's fish-or-coconuts choice are both micro questions. In Week 3, Demand, you'll meet the course's next model, the demand curve.

Pause and try this. Someone says the island model is useless because real islands have weather and storms. What would you say back?

Think about the bus map.

Here's the answer. Leaving storms out is what makes the model useful, the same way a bus map works because it skips the buildings. The model is built to answer one question about trade-offs, and it earns its place if its predictions hold up.

7. Sort the claims: positive versus normative statements

Economists make two very different kinds of claims, and a careful listener keeps them apart. Let's go to a city council meeting about the budget.

One council member says, "If we close the downtown library on weekends, fewer people will borrow books on weekends." Another says, "We should keep the library open on weekends." Those sound alike, but they're completely different kinds of claims.

A positive statement is a claim about what is, or what would happen, that can be checked against evidence, at least in principle. The first council member made one. The city could count weekend loans before and after the change and find out.

A normative statement is a claim about what ought to be, which rests on values and cannot be settled by evidence alone. The second council member made one. Two people could agree on every fact about library loans and still disagree about whether the library is worth the cost.

Here's the trap. In everyday talk, positive sounds good and normative sounds bad. In economics, that's not what they mean. Positive means testable. Normative means value-laden. A positive statement can even be false, and it's still positive, because evidence could show it's false.

Words like should, ought, better, and fair are clues to a normative claim. But the real test fits in one question: could evidence alone settle it? If yes, it's positive. If values decide it, it's normative.

The two kinds work together. Positive analysis tells the council what each choice would cost and what it would bring. The council's values then decide which choice to make.

Pause and try this. Label this one: "Repairing the main road this year would leave less money for new park benches."

Ask yourself whether the city's accounts could check it.

Here's the answer. It's positive. It's a claim about the budget's trade-off, and the city's own accounts can check whether the road money really comes out of the bench money.

Closing

Let's pull it together, in the order we built it. Scarcity means every choice is a trade-off, and opportunity cost is the single best thing you give up, which is why the concert costs 120 dollars and not 60. The production possibilities frontier draws that trade-off for a whole economy. Its slope is an opportunity cost, and because the island's costs climb from half a coconut to 3 coconuts a fish, the frontier bows outward. On the curve is efficient, inside is wasteful, and outside is out of reach. A shift of the whole curve outward is growth, while a slide along it is only a trade-off. An economic model leaves things out on purpose and holds all else equal. And a positive statement can be tested, while a normative one rests on values.

Next in this module, you'll find the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz. Next week, in Comparative Advantage, Specialization & the Gains from Trade, opportunity cost becomes the reason two people who trade can both come out ahead.

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