📖 经济学测试讲评

Economics

I. Lesson Focus

This lesson systematically reviews and deepens the core concepts of microeconomics and macroeconomics tested at CFA Level I. It covers demand and supply analysis with elasticity calculations, firm theory under perfect competition and monopoly, national income accounting (GDP), the money and banking system including the money multiplier, and international trade based on comparative advantage. The focus is on precise formula application, numerical problem solving, and avoiding common conceptual traps through detailed worked examples and targeted practice questions.

II. The Problem

CFA Level I candidates frequently lose marks on Economics due to errors in elasticity calculations, misidentification of shutdown versus break-even points, confusion between nominal and real GDP, incorrect application of the money multiplier, and mixing up absolute versus comparative advantage in trade scenarios. Typical exam questions require computing price elasticity using the midpoint method and determining revenue impact, calculating a firm’s break-even output, reconciling GDP via the expenditure and income approaches, determining the maximum change in money supply after an open-market operation, and identifying which country has a comparative advantage and the resulting gains from trade. This lesson teaches the underlying theory with formulas, works through three comprehensive numerical cases, highlights traps, and provides eight realistic practice questions with detailed solutions.

III. Demand, Supply, and Elasticity

Market demand curves slope downward; supply curves slope upward. Equilibrium occurs at their intersection.

Price Elasticity of Demand (PED) is defined as: $$ PED = \frac{\%\Delta Q_d}{\%\Delta P} = \frac{\Delta Q/Q}{\Delta P/P} $$ The midpoint (arc) formula is required to avoid inconsistency depending on the starting point: $$ PED = \frac{(Q_2 - Q_1)/((Q_2 + Q_1)/2)}{(P_2 - P_1)/((P_2 + P_1)/2)} $$ Interpretation: - |PED| > 1: elastic demand — price increase reduces total revenue - |PED| = 1: unit elastic - |PED| < 1: inelastic demand — price increase raises total revenue - |PED| = ∞: perfectly elastic (horizontal demand) - |PED| = 0: perfectly inelastic (vertical demand)

Income elasticity and cross-price elasticity are also examined to classify normal/inferior goods and substitutes/complements.

IV. Firm Theory and Profit Maximization

In perfect competition, firms are price takers and marginal revenue MR equals price P.

Profit-maximizing condition: MR = MC
Short-run shutdown point: P < minimum AVC
Break-even point: P = minimum ATC

Break-even quantity is calculated as: $$ Q_{BE} = \frac{FC}{P - AVC} $$ where FC is total fixed cost.

A monopolist faces a downward-sloping demand curve, so MR < P, but still maximizes profit where MR = MC, creating deadweight loss.

V. Macroeconomic Indicators: GDP and Price Level

Expenditure approach to GDP: $$ GDP = C + I + G + (X - M) $$ Income approach sums wages, interest, rent, profit, indirect taxes net of subsidies, and depreciation.

Nominal GDP uses current prices; real GDP uses base-year prices. The GDP deflator is: $$ GDP\ Deflator = \frac{Nominal\ GDP}{Real\ GDP} \times 100 $$

VI. Money and the Banking System

In the simple deposit multiplier model, the money multiplier is: $$ m = \frac{1}{Reserve\ Requirement\ Ratio\ (RRR)} $$ An increase in the monetary base ΔB leads to a maximum increase in money supply of m × ΔB.

The three main monetary policy tools are open-market operations, reserve requirements, and the discount rate.

VII. International Trade and Comparative Advantage

Absolute advantage exists when a country uses fewer resources to produce a good.
Comparative advantage exists when a country has a lower opportunity cost of producing a good.

Gains from trade arise from specialization according to comparative advantage. Tariffs and quotas both create deadweight loss and distort markets.

Worked Cases

Case 1: Price Elasticity and Pricing Strategy

A product has demand Qd = 1200 − 4P. At P = 150, Q = 600. The firm raises price to 165. Calculate PED using the midpoint method and determine the effect on total revenue.

Solution:
ΔP = 15, ΔQ = −60.
Midpoint PED = [−60 / ((600 + 540)/2)] / [15 / ((150 + 165)/2)] = (−60/570) / (15/157.5) ≈ −0.1053 / 0.0952 ≈ −1.106.
|PED| > 1 → elastic demand. Raising price reduces total revenue.
Original revenue = 150 × 600 = 90,000; new revenue = 165 × 540 = 89,100 (confirmed decline).

Case 2: Break-even and Shutdown Decision in Perfect Competition

A perfectly competitive firm has FC = 800, AVC = 5 + 0.02Q. Minimum AVC = 7 at Q = 100. Current price P = 12.

Calculate (1) break-even quantity and (2) whether the firm should shut down if P falls to 6.

Solution:
(1) At break-even, P = minimum ATC. Using the relation Q_BE = FC / (P − AVC) and AVC ≈ 7 near equilibrium:
Q_BE = 800 / (12 − 7) = 160 units.
(2) P = 6 < minimum AVC of 7 → firm should shut down immediately. Loss equals fixed costs of 800 only.

Case 3: Money Multiplier and GDP Accounting

The central bank purchases 100 billion in government bonds. RRR = 20 %, no excess reserves or cash drain. Compute maximum money-supply increase.
Separately, given C = 6,000 bn, I = 2,000 bn, G = 1,500 bn, X = 800 bn, M = 700 bn, depreciation = 300 bn, net indirect taxes = 200 bn, compute GDP by expenditure and income approaches.

Solution:
Money multiplier m = 1 / 0.2 = 5. Maximum money-supply increase = 5 × 100 = 500 billion.
Expenditure GDP = 6,000 + 2,000 + 1,500 + (800 − 700) = 9,600 billion.
Income-side GDP must equal 9,600 billion (wages + interest + rent + profit + net indirect taxes + depreciation), satisfying the accounting identity.

Traps

Key Formulas

• Midpoint PED = $\frac{(Q_2-Q_1)/((Q_2+Q_1)/2)}{(P_2-P_1)/((P_2+P_1)/2)}$
• Profit maximization: MR = MC
• Break-even quantity: $Q_{BE}=FC/(P-AVC)$
• Expenditure GDP: C + I + G + (X − M)
• GDP Deflator = (Nominal GDP / Real GDP) × 100
• Simple money multiplier: m = 1 / RRR
• Comparative advantage determined by lower opportunity cost

Practice Questions

Q1. A good’s price rises from 10 to 12 while quantity demanded falls from 200 to 150. Using the midpoint method, PED is closest to:
A. −0.82
B. −1.22
C. −1.45
D. −2.00

Q2. When |PED| > 1, an increase in price will:
A. Increase total revenue
B. Decrease total revenue
C. Leave total revenue unchanged
D. Have an indeterminate effect

Q3. In the short run, a perfectly competitive firm should shut down if price falls below:
A. ATC
B. AVC
C. AFC
D. MC

Q4. Nominal GDP is 12 trillion and the GDP deflator is 120 (base year = 100). Real GDP is closest to:
A. 10 trillion
B. 12 trillion
C. 14.4 trillion
D. 9.6 trillion

Q5. With a 12.5 % reserve requirement and no cash drain or excess reserves, the money multiplier equals:
A. 6
B. 8
C. 10
D. 12.5

Q6. Which of the following is NOT part of the expenditure approach to GDP?
A. Government purchases
B. Net exports
C. Transfer payments
D. Gross private domestic investment

Q7. Country A requires 2 hours to produce one X and 3 hours for one Y. Country B requires 3 hours for X and 4 hours for Y. Country A has a comparative advantage in:
A. X only
B. Y only
C. Both X and Y
D. Neither

Q8. The most direct economic consequence of an import tariff is:
A. Increased consumer surplus
B. Creation of deadweight loss
C. Reduced government revenue
D. Increased import volume

Answers

Takeaways

• Elasticity calculations must employ the midpoint formula; sign and magnitude have distinct economic meanings.
• Short-run firm decisions hinge on comparing price with AVC (shutdown) and ATC (break-even).
• Expenditure and income approaches to GDP are accounting identities; transfer payments are excluded.
• The simple money multiplier equals 1/RRR; real-world multipliers are smaller.
• Comparative advantage, not absolute advantage, determines the pattern and gains from trade.
• Both tariffs and quotas generate deadweight loss and reduce total welfare.