Economic concepts and models
This domain supplies the toolkit used across the Level 3 external: scarcity, opportunity cost, marginal analysis, the production possibility curve (PPC), circular flow and the positive/normative distinction. Questions reward using a model to explain a change, not merely describing it.
Core rules
- Scarcity forces choice: every choice has an opportunity cost - the value of the next best alternative forgone, not the money price.
- Ceteris paribus: models hold other influences constant so a single change can be traced through. State the assumption when you apply a model.
- Marginal reasoning: compare marginal benefit with marginal cost; efficient output is where MB = MC.
- PPC: points on the curve are efficient, inside are inefficient, outside are unattainable with current resources and technology. A bowed-out PPC shows increasing opportunity cost.
- Circular flow: households supply factors and buy goods; firms buy factors and sell goods. Injections (I, G, X) and withdrawals (S, T, M) change activity.
Worked example
A PPC shows 0 capital goods and 100 consumer goods, or 1 capital good and 90 consumer goods. The opportunity cost of the first capital good is 10 consumer goods. If the second capital good costs 20 consumer goods, the PPC is bowed out: resources suited to consumer goods become less productive in capital goods. Moving from 0 to 1 capital good sacrifices 10 consumer goods now but adds future production capacity - the growth trade-off.
Traps
- Calling the money paid the opportunity cost rather than the forgone alternative.
- Shifting the PPC when only demand changes; only resource quantity, quality or technology shifts it.
- Mixing positive (testable) and normative (value-judgment) statements.
- Describing circular flow without linking injections and withdrawals to income and output.