1. Unit 3—Aggregate Models
Krugman Section 4 Module 16
Graphs: 7
Time: 3 weeks

2. Consumption & Saving
What can a person do with disposable income (money earned after taxes)?
What is not spent is called savings
DI – C = Savings

3. Graphs Consumption in notes Savings in notes Consumption Savings Link
Big graph number 4

4. Average Propensities to C & S
Measures the average C (APC) or S (APS) at any level of disposable income
APC = C / DI
APS = S / DI
C% ↓ and S% ↑ as DI ↑
APC + APS = 1

5. Marginal Propensities to C & S
(marginal means extra)
Proportion of any change in income C is called MPC or income Saved is called MPS
MPC = ∆ C / ∆ DI
MPS = ∆ S / ∆ DI
MPC + MPS = 1
The only choice people have is to C or to Save. An additional dollar in income must result in a change in C and/or a change in Savings.

6. Practice Worksheet
APC/APS
MPC/MPS

7. Investment
Spending on new plants, capital equipment, machinery, construction, etc.
Investment decision weighs mb & mc
The expected rate of return = mb
The interest rate = mc

8. Expected Rate of Return
Found by comparing the expected economic profit (total revenue minus total cost) to cost of investment to get expected rate of return
Woodworker wants to buy equipment for $1, He expects a $100 profit. The expected rate of return is 10%. In order to make a profit, the woodworker would not want to pay more than 10% interest on the investment.

9. Investment Demand Curve
Movement occurs with a change in the interest rate
Shifts occur due to these determinants:
1. operating costs
When cost falls, the r from prospective investment project rises, shifts the IDC to the right
Higher electricity costs = shift to the left
2. business taxes
↑in taxes = shift to the left

10. Shifts Continued 3. technological change 4. stock on hand (inventory)
Development stimulates investment
(shifts to the right)
4. stock on hand (inventory)
When firms are overstocked, the r declines (shifts to the left)
There is little incentive to invest in new capital when there is excess production
When firms are under stocked, the r increases (shifts to the right)

11. Shifts continued again . . .
5. Expectations
Optimistic about future sales, the curve will shift to the right
Pessimistic outlook = shift to the left

12. Investment Demand Curve
BIG GRAPH #5