One picture holds scarcity, trade-offs, opportunity cost and efficiency at once — which is why it keeps coming back all quarter. The marks are in reading points off it correctly and knowing what moves the curve versus what moves you along it.
The production possibilities curve shows every combination of two goods an economy can make with all its resources and current technology. Most marks come from classifying a point correctly.
| Where the point sits | Called | What it means |
|---|---|---|
| On the curve | Efficient & attainable | Every resource is used and used well. More of one good requires less of the other. |
| Inside the curve | Inefficient but attainable | Something is idle — unemployment, an unused factory. More of both is possible. |
| Outside the curve | Unattainable | Not reachable today. It needs the curve itself to move. |
| On an axis | Complete specialisation | All resources in one good. Efficient, and usually a terrible idea. |
Every point on the curve is efficient, and the curve has infinitely many points. Economics cannot tell you which one to pick — that needs preferences, and preferring one is a normative claim. "Efficient" means no waste, not "right."
A straight PPC means constant opportunity cost. A bowed one means it rises — and the reason is worth a sentence in any written answer.
Resources are not equally suited to both goods. Move the first workers from bread to laptops and you move the ones who were bad at baking anyway — cheap. Keep going and you start conscripting your best bakers, who give up a lot of bread per laptop gained. Each extra laptop costs more bread than the last. That is increasing opportunity cost, and it is what bends the line into a curve.
Drag along the frontier below. The slope is the opportunity cost, and it steepens as you go.
Moving along the curve is a choice. Moving the curve is growth. Exams test the difference constantly, and they love technology that only helps one good.
The second model separates balanced growth from one-sided growth, which is where the marks hide.
Four shapes, and the error each is hunting for.
| Asked as | Do this | Trap |
|---|---|---|
| "Classify point Z" | On / inside / outside. Say what each implies. | Calling an inside point "unattainable" — it is attainable and wasteful. |
| "Opportunity cost of moving A→B" | Subtract: how much of the other good was lost. | Giving the new total rather than the change. |
| "Does this shift the curve?" | New resources or technology → yes. A different choice → no. | Treating trade as a shift. It moves consumption, not production possibilities. |
| "Why is it bowed?" | Resources are not equally suited to both goods. | Answering "because of scarcity" — true of the whole curve, not its shape. |
Shift-versus-movement is the exact distinction that decides supply-and-demand questions next chapter. There, a change in the good's own price moves you along a curve; anything else shifts it. Same logic, new axes.
Shuffled, not blocked. Several items give you only a described point and ask you to classify it.
| Model | Genuinely computed | Simplified |
|---|---|---|
| The frontier | The curve itself, the maximum bread at any laptop count, the opportunity cost of the next unit by differencing the frontier, and the classification of your point. | A quarter-ellipse stands in for the real frontier. Real economies have kinked, lumpy ones. |
| Growth | Both intercepts after resource and technology changes, the shift type, and whether a fixed previously-unreachable point is now inside the new frontier. | Resource growth scales both goods equally; technology here affects one good only. |