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MICROECONOMICS · UNIT 1The shape of scarcity
Four tools that explain almost everything in a first economics course — and the small mistakes that quietly ruin each one.
Every introductory economics course opens with the same claim: resources are limited, wants are not, and so every choice costs something. That claim sounds obvious until you try to draw it. Then it turns out that most of the difficulty in early economics is not the ideas — it is reading the picture correctly.
What follows is the set of tools the first unit rests on: opportunity cost, the rationing problem, the production possibilities frontier, comparative advantage, and the humble two-variable graph. Each comes with a worked example, a diagram, and the specific error students make with it most often. Five self-test questions are waiting at the end.
One cost, many trade-offs
Three words do most of the work here, and they are not interchangeable.
- Scarcity — a resource is limited relative to the uses you have for it. Time, money and energy are all scarce for most people most of the time.
- Trade-off — any alternative you give up when you choose one option. There are usually several.
- Opportunity cost — the value of the best alternative forgone. There is only ever one.
A worked example
Priya has eight free hours on a Saturday. She can work a shift at her part-time job for $160, study for a midterm worth 30% of her grade, drive her younger brother to practice, or simply rest after a heavy week. She studies.
The scarce resource is the eight hours, limited because a day has a fixed length and the rest of hers is already committed to classes, commuting and sleep. The trade-offs are the shift, the driving and the rest. The opportunity cost is the $160 shift alone, because that was the next-best use of the time. The driving and the rest remain trade-offs; they are not the cost.
"The opportunity cost was the eight hours." The hours are the resource spent, not the alternative forgone. A cost in this sense is always a thing you could have had instead.
The same scarcity, three different agents
Scarcity of time is not only a student's problem. The identical constraint reappears at every scale, and noticing that is half of what economics teaches.
- A household has a fixed number of adult hours a week to split between paid work, childcare and unpaid labour at home. More overtime means less time with the children.
- A firm has a fixed number of labour hours it has already paid for, to assign to production, training or maintenance. Training staff today means fewer units shipped today.
- A government has a fixed legislative calendar and a fixed civil service. Debating one bill at length means another never reaches a vote this session.
Who decides, and what does the rationing
Every economic system answers the same three questions — what to produce, how to produce it, and for whom. What differs is who answers them, and what mechanism settles the shortage that scarcity guarantees.
| Market | Command | Mixed | |
|---|---|---|---|
| What to produce | Consumers, through what they buy | Central planners, through a state plan | Mostly consumers; the state sets priorities in some sectors |
| How to produce | Firms, seeking the lowest cost | State enterprises, following plan targets | Firms, inside regulation on safety, labour and environment |
| For whom | Whoever can pay the price | Whoever the plan allocates to | Price for most goods; universal access for some services |
| Rationed by | Price. Shortages raise it until quantity demanded falls | Quota, queue or ration card. Price is held fixed, so queues appear | Price mostly; waiting lists and eligibility rules where access is universal |
Public transit in most Canadian cities shows the mixture plainly. The service is publicly owned and subsidised, so the fare does not cover the cost of a ride, and a public body decides which routes exist at all. Yet riders choose freely between transit, driving and cycling, and private firms build the buses. Scarcity is rationed partly by price — the fare — and partly by waiting: a bus comes every twelve minutes, so riders queue.
Hospital care rationed by triage rather than by price, electricity sold at a regulated rate, and carbon pricing used to shift private behaviour without banning any product are all the same hybrid in different clothes. No system abolishes scarcity. Each one simply decides who goes without, and by what rule.
Reading a production possibilities frontier
A production possibilities frontier shows every combination of two goods you can produce when your resources are fully used. The curve is a boundary, and nearly everything the diagram has to say is contained in where a point sits relative to it.
- On the curve (A). Every hour is used. The only way to get more study time is to give up paid work. This is what efficiency means here — not working hard, but leaving nothing idle.
- Inside the curve (B). More of both goods is available. Hours are idle, or badly used. Unemployment lives here.
- Outside the curve (C). Desirable and impossible, at least for now. Growth is the business of moving the boundary, not the point.
Why it bows outward
The curve is concave because of increasing opportunity cost. The first hours moved from work to study come out of the least productive shifts, so little is lost. The last hours come out of the best-paid ones, so a great deal is. You can read this straight off the graph. Her first five study hours cost her only 1 hour of paid work (40 down to 39). The next five cost 2, then 3, then 5, then 7 — and the five hours between 25 and 30 cost her 9 (22 down to 13). Same five hours of studying each time, nine times the sacrifice by the end.
Resources are not perfectly interchangeable between uses. That single fact is what bends the line. Where resources are interchangeable — where the trade is one-for-one all the way along — the frontier is a straight line instead, and we will see one shortly.
Shifting the boundary, and tilting it
Students describe any improvement as "the curve shifts out." Sometimes it does. Often it rotates, and the difference is not cosmetic — it changes which goods become cheaper relative to which.
- Does the change make both goods easier to produce? The whole curve shifts outward. More workers, more capital, better general education, better public health.
- Does it make one good easier to produce? The curve rotates away from that good's axis, and the other intercept stays exactly where it was. A machine that only speeds up laptop assembly does nothing whatever for tablet output.
- Does it destroy resources? The curve shifts inward. A factory fire, an ageing workforce, a failed harvest.
"Working harder" is not a shift. It moves you from inside the curve to a point on it. Growth and recovery look similar on a chart of output and are completely different things.
When the trade is constant: a straight frontier
Not every frontier bends. If resources move between two uses at a fixed rate, opportunity cost is constant and the boundary is a straight line. Here is a small bakery working a sixty-hour week: it can turn out 30 cakes and no bread, or 120 loaves and no cakes, or any mix in between.
The slope carries the whole story. One cake costs 120 ÷ 30 = 4 loaves; one loaf costs 30 ÷ 120 = 0.25 cakes. At 15 cakes the bakery could still bake 120 − (15 × 4) = 60 loaves, so producing only 40 puts it inside the boundary with twenty loaves of capacity going to waste. At 20 cakes the ceiling is 120 − (20 × 4) = 40 loaves, so asking for 60 is simply beyond the week.
Both shapes — bowed and straight — are correct frontiers for different situations. What matters is that the shape you draw agrees with the story you tell beside it.
Being worse at everything, and still worth trading with
Absolute advantage is about who produces more. Comparative advantage is about who gives up less. Only the second one decides who should specialise, and this is where the subject stops being obvious.
The production table
Alina and Bruno each run a food stall and each works an eight-hour day. Using that same day, either could produce either good:
| Producer | Pies per day | Salads per day |
|---|---|---|
| Alina | 24 | 48 |
| Bruno | 20 | 60 |
Alina makes more pies, so she holds the absolute advantage there. Bruno makes more salads, so he holds it in salads. Now price each good in terms of the other, by dividing the other good's output by this one's:
| Producer | Cost of one pie | Cost of one salad |
|---|---|---|
| Alina | 48 ÷ 24 = 2 salads | 24 ÷ 48 = 0.5 pies |
| Bruno | 60 ÷ 20 = 3 salads | 20 ÷ 60 = 0.33 pies |
Alina sacrifices 2 salads per pie against Bruno's 3, so pies are hers. Bruno sacrifices 0.33 pies per salad against Alina's 0.5, so salads are his. The comparative advantages always land on opposite goods; if your arithmetic hands one producer both, the division went wrong somewhere.
Where the gain comes from
Suppose each splits the day evenly before specialising, then each produces only their comparative-advantage good:
| Pies | Salads | |
|---|---|---|
| Each splits the day | 12 + 10 = 22 | 24 + 30 = 54 |
| Each specialises | 24 | 60 |
| Gain | +2 | +6 |
Two more pies and six more salads, from no extra hours, no new equipment and no one working harder. The gain is purely the result of each hour going to whoever sacrifices least by using it. For trade itself to leave both better off, the exchange rate must sit between the two opportunity costs — somewhere between 2 and 3 salads per pie. At 2.5, Alina receives more salads for a pie than she could have made herself, and Bruno parts with fewer salads than making the pie would have cost him.
A producer can be worse at making everything and still have something worth trading. Being outclassed in absolute terms says nothing about where your hours do the least damage. That asymmetry is the whole argument for specialisation, and it scales from two market stalls to two economies without changing shape.
The plain two-variable graph
The least glamorous tool in the unit is also the one most often botched. Before anyone considers the shape, a graph of your own data has to carry three things: both variables named, both units given, and every observation plotted — including the awkward ones.
Describe the relationship with one of three words, and then say why. Positive: the variables move together, and the points trend upward left to right. Negative: they move in opposite directions, and the points trend down. Unclear: the points scatter with no visible direction, or the direction reverses partway through.
Then name a limitation, and make it a real one. "My data might be wrong" is not an answer. A sample of seven cannot rule out coincidence. Hours recalled at the end of a week are rounded toward what felt right. Quiz difficulty changed from week to week, so some of the movement has nothing to do with studying at all. And weeks with more study time were probably also weeks with less paid work and more sleep — in which case the sleep may be doing the work the graph credits to the studying.
A point that breaks the pattern is evidence, not an embarrassment. Mentioning it and explaining what happened that week is better economics than quietly deleting the row.
Five questions to test yourself
Work each one through before opening the answer. These are the specific places where understanding and recognition come apart.
01Devon's $400
Devon has $400 saved. He can buy a used laptop for coursework, put the money toward car repairs, or keep it as an emergency buffer. He buys the laptop. Name the scarce resource, two trade-offs, and the opportunity cost.
Answer
The scarce resource is the $400, limited because it is what he has saved from part-time work and nothing more is coming before the term ends. The trade-offs are the car repairs and the emergency buffer. The opportunity cost is whichever of the two he valued most — say the repairs, because without them his commute is at risk. Only one alternative can be the opportunity cost; the other stays a trade-off.
The frequent slip is answering "$400." That is the resource spent, not the alternative forgone.
02The bread queue
In a command economy, bread is priced below the cost of making it and queues form outside bakeries every morning. What is doing the rationing, and what would happen to the queue in a market economy?
Answer
The queue is doing the rationing. Because price is held below the market-clearing level, quantity demanded exceeds quantity supplied, and the shortage is settled by who is willing to wait rather than who is willing to pay. In a market economy price would rise until quantity demanded matched supply, so the queue would shorten and price would do the rationing instead.
Neither system removes the scarcity. Each one excludes a different group of people — and which exclusion you find tolerable is a political question, not an economic one.
03Points on and off a frontier
A workshop can produce 40 chairs and no tables, or 10 tables and no chairs, with constant opportunity cost. (a) What does one table cost? (b) Is 5 tables and 25 chairs attainable? Efficient? (c) Is 8 tables and 15 chairs attainable?
Answer
(a) 40 ÷ 10 = 4 chairs per table. Going the other way, one chair costs 10 ÷ 40 = 0.25 tables.
(b) At 5 tables the workshop could still make 40 − (5 × 4) = 20 chairs. It is producing 25, which is more than the frontier allows at that point — so this combination is unattainable, not merely inefficient. Check the arithmetic before you check the picture; the eye is easily fooled near the line.
(c) At 8 tables the ceiling is 40 − (8 × 4) = 8 chairs. Fifteen chairs is well beyond it, so no.
04Teo and Mira
In one day, Teo can knit 6 scarves or 12 hats. Mira can knit 10 scarves or 15 hats. Who holds the absolute advantage in each good? Who should specialise in what?
Answer
| Producer | Scarves | Hats | Cost of 1 scarf | Cost of 1 hat |
|---|---|---|---|---|
| Teo | 6 | 12 | 12 ÷ 6 = 2 hats | 6 ÷ 12 = 0.5 scarves |
| Mira | 10 | 15 | 15 ÷ 10 = 1.5 hats | 10 ÷ 15 = 0.67 scarves |
Mira holds the absolute advantage in both goods — she out-produces Teo in scarves (10 to 6) and in hats (15 to 12). That settles nothing.
A scarf costs Mira 1.5 hats but costs Teo 2, so scarves are Mira's. A hat costs Teo 0.5 scarves but costs Mira 0.67, so hats are Teo's. They should trade at a rate between 1.5 and 2 hats per scarf. Teo, slower at everything, still gives up less to make a hat, and that is the only thing that counts.
05Work and sleep
A student records her weekly paid-work hours and her sleep. As work rises from 5 hours to 25, sleep falls from 56 hours to 41. In week four she worked 20 hours and slept 52. Describe the relationship and name one limitation.
Answer
The relationship is negative: as paid work rises, sleep falls, so the points trend downward from left to right. Week four sits well above the trend and should be named rather than dropped — something else was going on, perhaps a reading week with no classes.
One real limitation: with a handful of self-reported weeks, a third variable such as assignment deadlines could be driving both series at once. Heavy deadline weeks may push work hours down and sleep down together, which weakens the simple story that work hours cause the sleep loss.
A short checklist for your own graphs
- Both axes labelled, with units — "hours per week", not "study".
- Numbers that add up to something real. If two activities sum to 90 hours in a seven-day week, the graph is telling on you.
- Every opportunity cost shown as a division, not just a result. The arithmetic is where the credit is.
- A shape that matches the explanation beside it — bowed for increasing cost, straight for constant.
- Points that break the pattern left in, and accounted for.
Reading a frontier is one thing; dragging one until the trade-offs bite is another.
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