1. 2 chapter: >> Economic Models: Trade-offs and Trade
Krugman/Wells
©2009  Worth Publishers 1

2. Why models? Simplified representations of reality—play a crucial role in economics
Two simple but important models:
production possibility frontier
circular-flow diagram
The difference between positive economics and normative economics
When economists agree and why they sometimes disagree

3. Models in Economics
A model is a simplified representation of a real situation that is used to better understand real-life situations.
Create a real but simplified economy
Ex.: Cigarettes in World War II prison camps
Simulate an economy on a computer
Ex.: Tax models, money models…
The “other things equal” assumption means that all other relevant factors remain unchanged.

4. Trade-offs: The Production Possibility Frontier
The production possibility frontier (PPF) illustrates the trade-offs facing an economy that produces only two goods. It shows the maximum quantity of one good that can be produced for any given production of the other.
The PPF improves our understanding of trade-offs by considering a simplified economy that produces only two goods by showing this trade-off graphically.

5. The Production Possibility Frontier
Quantity of coconuts 30 D
Feasible and
efficient
in production
Not feasible A 15
Feasible but
not efficient B 9 C
Production possibility frontier
PPF 20 28 40
Quantity of fish

6. Increasing Opportunity Cost
Quantity of coconuts
Producing the first
20 fish . . .
…requires giving up
5 coconuts 35 30
But producing
20 more fish . . . A 25 20
…requires giving up
25 more coconuts… 15 10 5
PPF 10 20 30 40 50
Quantity of fish

7. Economic Growth
Quantity of coconuts
Production is initially at point A (20 fish and 25 coconuts),  it can move to point E (25 fish and 30 coconuts).
The economy can now produce more of everything.
Economic growth results in an outward shift of the PPF because production possibilities are expanded. 35 E 30 A 25 20 15 10 5
Original
New
PPF
PPF 10 20 25 30 40 50
Quantity of fish

8. Production Possibilities for Two Castaways
(a) Tom’s Production Possibilities
Quantity of coconuts 30
Tom’s consumption
without trade 9
Tom’s
PPF 28 40
Quantity of fish

9. Production Possibilities for Two Castaways
(a) Hank’s Production Possibilities
Quantity of coconuts 20
Hank’s consumption
without trade 8
Hank’s
PPF 6 10
Quantity of fish

10. Tom and Hank’s Opportunity Costs
Tom’s Opportunity Cost
Hank’s Opportunity Cost
One fish
3/4 coconut
2 coconuts
One coconut
4/3 fish
1/2 fish

11. Specialize and Trade
Both castaways are better off when they each specialize in what they are good at and trade.
It’s a good idea for Tom to catch the fish for both of them, because his opportunity cost of a fish in terms of coconuts not gathered is only 3/4 of a coconut, versus 2 coconuts for Hank.
Correspondingly, it’s a good idea for Hank to gather coconuts for the both of them.

12. Comparative Advantage and Gains from Trade
(a) Tom’s Production and Consumption
(b) Hank’s Production and Consumption
Quantity of coconuts
Quantity of coconuts 30
Tom’s consumption without trade
Hank’s production with trade
Tom’s consumption with trade 20
Hank’s consumption with trade
Tom’s production with trade 10 10
Hank’s consumption without trade 9 8 T o m 's
Hank's
PPF
PPF 28 30 40 6 10
Quantity of fish
Quantity of fish

13. How the Castaways Gain from Trade
Both Tom and Hank experience gains from trade:
Tom’s consumption of fish increases by two, and his consumption of coconuts increases by one.
Hank’s consumption of fish increases by four, and his consumption of coconuts increases by two.

14. Comparative vs. Absolute Advantage
An individual has a comparative advantage in producing a good or service if the opportunity cost of producing the good is lower for that individual than for other people.
An individual has an absolute advantage in an activity if he or she can do it better than other people. Having an absolute advantage is not the same thing as having a comparative advantage.

15. Tom vs. Hank – Absolute vs. Comparative
Tom has an absolute advantage in both activities: he can produce more output with a given amount of input (in this case, his time) than Hank.
But we’ve just seen that Tom can indeed benefit from a deal with Hank because comparative, not absolute, advantage is the basis for mutual gain.
So Hank, despite his absolute disadvantage, even in coconuts, has a comparative advantage in coconut gathering.
Meanwhile Tom, who can use his time better by catching fish, has a comparative disadvantage in coconut-gathering.

16. Comparative Advantage and International Trade
(a) The U.S. Production Possibilities Frontier
(b) Canadian Production Possibilities Frontier
Quantity of aircraft
Quantity of aircraft
3,000
Canadian production with trade
U.S. consumption without trade
Canadian consumption without trade
U.S. consumption with trade
2,000
1,500
1,500
U.S. production with trade
Canadian consumption with trade
1,000 U
.S.
Canadian
PPF
PPF 1 2 3
0.5 1
1.5
Quantity of pork (millions of tons)
Quantity of pork (millions of tons)

17. Comparative Advantage and International Trade
Just like the example of Tom and Hank, the U.S. and Canada can both achieve mutual gains from trade.
If the U.S. concentrates on producing pork and ships some of its output to Canada, while Canada concentrates on aircraft and ships some of its output to the U.S., both countries can consume more than if they insisted on being self-sufficient.

18. Misunderstanding Comparative Advantage
A common mistake is to confuse comparative advantage with absolute advantage.
Ex.: U.S. vs. Japan in 1980s:
Commentators: “U.S. might soon have no comparative advantage in anything”
Wrong! They meant “absolute advantage”

19. Transactions: The Circular-Flow Diagram
Trade takes the form of barter when people directly exchange goods or services they have for goods or services they want.
The circular-flow diagram is a model that represents the transactions in an economy by flows around a circle.

20. The Circular-Flow Diagram
Money
Households
Money
Goods
Factors
and
services
Markets for goods and services
Factor Markets
Goods
and
Factors
services
Firms
Money
Money

21. Circular-Flow of Economic Activities
A household is a person or a group of people that share their income.
A firm is an organization that produces goods and services for sale.
Firms sell goods and services that they produce to households in markets for goods and services.
Firms buy the resources they need to produce—factors of production—in factor markets.

22. Circular-Flow of Economic Activities
Ultimately, factor markets determine the economy’s income distribution, how total income is divided among the owners of the various factors of production.

23. Using Models
Positive economics is the branch of economic analysis that describes the way the economy actually works.
Normative economics makes prescriptions about the way the economy should work.
A forecast is a simple prediction of the future.

24. Using Models
Economists can determine correct answers for positive questions, but typically not for normative questions, which involve value judgments.
The exceptions are when policies designed to achieve a certain prescription can be clearly ranked in terms of efficiency.
It is important to understand that economists don’t use complex models to show “how clever they are,” but rather because they are “not clever enough” to analyze the real world as it is.

25. When and Why Economists Disagree
There are two main reasons economists disagree:
Which simplifications to make in a model
Values

26. Almost all economics is based on models
Almost all economics is based on models. An important assumption in economic models is the other things equal assumption, which allows analysis of the effect of a change in one factor by holding all other relevant factors unchanged.
One important economic model is the production possibility frontier. It illustrates: opportunity cost, efficiency, and economic growth. There are two basic sources of growth: an increase in factors of production, resources such as land, labor, capital, and human capital, inputs that are not used up in production, and improved technology.

27. Another important model is comparative advantage, which explains the source of gains from trade between individuals and countries. Everyone has a comparative advantage in something. This is often confused with absolute advantage, an ability to produce a particular good or service better than anyone else.
In the simplest economies, people barter or trade goods and services for one another—rather than trade them for money, as in a modern economy. The circular-flow diagram represents transactions within the economy as flows of goods, services, and money between households and firms. These transactions occur in markets for goods and services and factor markets.

28. Economists use economic models both for positive economics, which describes how the economy works, and for normative economics, which prescribes how the economy should work. Positive economics often involves making forecasts. Economists can determine correct answers for positive questions, but typically not for normative questions, which involve value judgments.
There are two main reasons economists disagree. One, they may disagree about which simplifications to make in a model. Two, economists may disagree—like everyone else—about values.

29. Coming attraction Chapter 3: Supply and Demand
The End of Chapter 2
Coming attraction Chapter 3: Supply and Demand 31