📖 市场结构周测（10 题）

Economics

I. Lesson Focus

This lesson consolidates the core concepts of market structures tested heavily on the CFA Level I exam. Candidates must master the characteristics of perfect competition, monopolistic competition, oligopoly, and monopoly; profit-maximization rules; short-run versus long-run equilibrium; efficiency analysis; and government regulation of monopolies. The session reviews formulas, works through detailed numerical examples, highlights common traps, and provides eight rigorous practice questions with full explanations.

II. The Problem

Market structure questions appear frequently on the CFA exam in both vignette and standalone formats. Candidates often confuse the conditions for long-run economic profit, the relationship between price and marginal revenue, the presence of excess capacity, deadweight loss calculations, and the implications of natural monopoly regulation. This weekly quiz reviews the underlying theory while testing application through calculation and scenario analysis.

III. Classification and Characteristics of Market Structures

Market structure is classified along four key dimensions: number of firms, degree of product differentiation, barriers to entry and exit, and the firm’s ability to influence price.

• Perfect Competition: Many firms, homogeneous products, no barriers to entry, price takers. The demand curve facing the firm is perfectly elastic: P = MR = AR.
• Monopolistic Competition: Many firms, differentiated products, low barriers, limited pricing power. Long-run economic profit is zero, but firms engage in non-price competition (branding, advertising).
• Oligopoly: Few firms, homogeneous or differentiated products, high barriers, interdependence among competitors. Common models include Cournot, Bertrand, Stackelberg, and cartel behavior.
• Monopoly: Single seller, unique product, very high barriers, price maker. The firm faces the entire market demand curve; its MR curve lies below the demand curve.

IV. Profit-Maximization Condition and Short-Run versus Long-Run Equilibrium

In all market structures, firms maximize profit where MR = MC.

• Perfect Competition: Short-run economic profits, losses, or break-even are possible. In the long run, free entry and exit drive economic profit to zero at the minimum point of ATC, achieving both productive efficiency (P = minimum ATC) and allocative efficiency (P = MC).
• Monopolistic Competition: Short-run economic profits are possible. In the long run, entry shifts the demand curve left until P = ATC (tangent), but P > MC, resulting in excess capacity. Neither productive nor allocative efficiency is achieved.
• Oligopoly: Outcomes depend on the model. Cartels attempt to replicate monopoly profits but suffer from the “prisoner’s dilemma” and incentive to cheat.
• Monopoly: High barriers allow positive economic profit to persist in the long run. Deadweight loss exists because P > MC. In a natural monopoly, economies of scale cause ATC to decline over the relevant output range; one firm can supply the market at lowest cost. Governments often regulate by setting price caps at the intersection of demand with ATC or MC.

Relationship between MR and Price: - Perfect competition: MR = P - Imperfect competition: MR < P; the gap widens as the demand curve becomes steeper.

V. Efficiency Analysis and Government Intervention

• Allocative Efficiency: Resources are allocated optimally when P = MC.
• Productive Efficiency: Goods are produced at the lowest possible cost when P = minimum ATC.
• Perfect competition achieves both in long-run equilibrium. Other structures generally do not, unless regulated.
• Government responses to monopoly include antitrust laws, price ceilings, marginal-cost pricing, average-cost pricing, subsidies, or nationalization.
• Consumer surplus and producer surplus are both reduced under monopoly, creating a deadweight loss triangle.

Worked Cases

Case 1: Long-Run Equilibrium in Perfect Competition

In a perfectly competitive industry the market price is $15. A representative firm has MC = 2Q + 5 and ATC = Q² + 5Q + 10. Find the long-run equilibrium output and determine whether economic profit exists.

Solution: Long-run equilibrium requires P = MR = MC = minimum ATC.
Set MC = P: 2Q + 5 = 15 → Q = 5.
Substitute into ATC: ATC = 25 + 25 + 10 = 60.
Since P ($15) < ATC ($60), firms incur losses in the short run. In the long run, exit occurs until price rises to the minimum attainable ATC. The example illustrates that simply equating MC and P does not guarantee zero economic profit; the long-run condition is P = minimum ATC.

Case 2: Monopoly Pricing and Deadweight Loss

A monopolist faces demand P = 100 – 2Q and constant MC = 20 (no fixed costs). Calculate the profit-maximizing price and quantity, and the resulting deadweight loss.

Solution: TR = 100Q – 2Q² → MR = 100 – 4Q.
Set MR = MC: 100 – 4Q = 20 → Q = 20.
P = 100 – 2(20) = $60.
Competitive equilibrium (P = MC): 100 – 2Q = 20 → Q = 40, P = $20.
Deadweight loss = ½ × (40 – 20) × (60 – 20) = ½ × 20 × 40 = $400.
The triangle represents lost surplus due to underproduction.

Case 3: Regulation of a Natural Monopoly

A natural monopolist has ATC = 500/Q + 10 and MC = 10. Market demand is P = 100 – 0.5Q. If regulators impose average-cost (fair-return) pricing, find equilibrium quantity and price.

Solution: Set P = ATC: 500/Q + 10 = 100 – 0.5Q.
Multiply through by Q: 500 + 0.5Q² = 90Q.
Rearrange: 0.5Q² – 90Q + 500 = 0 → Q² – 180Q + 1,000 = 0.
Solving the quadratic yields Q ≈ 5.7 (discard negative root).
P ≈ 100 – 0.5(5.7) ≈ $97.15.
At this point P = ATC > MC, so some deadweight loss remains, but the outcome is superior to unregulated monopoly pricing.

Traps

Key Formulas

• Profit-maximization (all structures): $MR = MC$
• Perfect competition (long run): $P = MR = MC = \text{minimum ATC}$
• Monopoly: $MR = P(1 + 1/E_d)$
• Deadweight loss (monopoly): $\frac12(Q_c - Q_m)(P_m - MC)$
• Lerner Index: $\frac{P - MC}{P} = -\frac{1}{E_d}$
• Concentration measures: 4-firm concentration ratio, Herfindahl-Hirschman Index (HHI)
• Excess capacity (monopolistic competition): $Q_{\text{ATCmin}} - Q_{\text{actual}}$

Practice Questions

Q1. In a perfectly competitive market, a firm’s short-run supply curve is the portion of the:
A. ATC curve above AVC.
B. MC curve above AVC.
C. MR curve.
D. ATC curve to the right of its minimum.

Q2. Which of the following is least likely a characteristic of a monopoly?
A. Barriers to entry exist.
B. The MR curve lies below the demand curve.
C. Long-run economic profit equals zero.
D. The firm faces the market demand curve.

Q3. In long-run equilibrium, a monopolistically competitive firm will:
A. Produce at the minimum of its ATC curve.
B. Set P = MC.
C. Operate with excess capacity.
D. Earn positive economic profit.

Q4. An oligopolist faces a more elastic demand if rivals do not match a price cut and a less elastic demand if rivals do match. This description corresponds to which model?
A. Cournot model.
B. Kinked demand curve model.
C. Bertrand model.
D. Cartel model.

Q5. A natural monopoly is most likely to occur when:
A. Fixed costs are low and ATC rises with output.
B. Significant economies of scale cause ATC to decline over the relevant range.
C. Products are highly differentiated.
D. Barriers to entry are very low.

Q6. If a firm’s Lerner Index equals 0.4, the absolute value of the price elasticity of demand it faces is closest to:
A. 0.4.
B. 1.5.
C. 2.5.
D. 4.0.

Q7. The main problem with regulators forcing a monopolist to set price equal to marginal cost is that the firm will:
A. Generate large deadweight loss.
B. Incur economic losses and require subsidy.
C. Create excess capacity.
D. Increase consumer surplus but reduce producer surplus.

Q8. Which market structure is most likely to achieve both productive and allocative efficiency?
A. Monopoly.
B. Monopolistic competition.
C. Oligopoly.
D. Perfect competition in long-run equilibrium.

Answers

Takeaways

• MR = MC is the universal profit-maximization rule, but the MR–P relationship differs across structures.
• Perfect competition is the only structure that achieves both productive and allocative efficiency in long-run equilibrium.
• Imperfectly competitive markets generate deadweight loss and allocative inefficiency.
• Natural monopolies require regulation; average-cost pricing is a realistic compromise that avoids losses.
• Oligopoly is defined by strategic interdependence; cartels are unstable due to cheating incentives.
• The Lerner Index quantifies market power and is inversely related to demand elasticity.