Imperfect Competition — AP Microeconomics Study Guide

For: AP Microeconomics candidates sitting AP Microeconomics.

Covers: The full Unit 4 Imperfect Competition syllabus, including introduction to imperfect markets, monopoly, price discrimination, monopolistic competition, oligopoly, and game theory, with core rules for all imperfect competition models.

You should already know: The profit maximization rule $MR=MC$ for perfectly competitive firms, the definition of economic vs accounting profit, consumer and producer surplus and deadweight loss analysis.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Microeconomics style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. Why This Matters (Whole Unit)

Imperfect Competition is one of the most heavily weighted units on the AP Microeconomics exam, accounting for 14-18% of your total exam score, and appears regularly in both multiple-choice and free-response questions, often as the core of a long FRQ combined with market failure analysis.

Unlike the benchmark model of perfect competition, which is a useful theoretical starting point, almost all real-world markets are imperfectly competitive. You encounter these markets every day: the local electric utility (monopoly), your neighborhood coffee shops (monopolistic competition), domestic airlines (oligopoly), and pharmaceutical companies selling patented drugs (monopoly with price discrimination). This unit teaches you how firms with market power (the ability to set their own price above marginal cost) make decisions, why these decisions often lead to inefficient outcomes, and how public policy can address these inefficiencies.

The core ideas of this unit build directly on what you learned about perfect competition, and lay the foundation for analyzing firm behavior, market failure, and public policy in later topics. This unit also introduces game theory, a core tool for analyzing strategic interaction that is used in many subfields of economics.

2. Concept Map: How The 5 Unit Sub-Topics Build On Each Other

The five sub-topics of this unit build sequentially, starting from foundational definitions and moving to more complex, real-world market structures:

1. Introduction to Imperfectly Competitive Markets: This first sub-topic establishes the core unifying feature of all imperfect competition: market power. Unlike perfectly competitive firms that are price takers facing a horizontal demand curve, all imperfectly competitive firms face a downward-sloping market demand curve, so marginal revenue is less than price. This foundation applies to every other model in the unit.
2. Monopoly: The simplest model of imperfect competition, with a single firm facing no direct competition, protected by high barriers to entry. This sub-topic teaches the core profit maximization procedure (find $Q$ where $MR=MC$, set $P$ from the demand curve) and deadweight loss analysis that you will reuse in all other imperfect competition models.
3. Price Discrimination: An extension of monopoly that explores how firms with market power can increase their profit by charging different prices to different customers for the same good, rather than charging a single uniform price. This builds directly on the monopoly profit maximization framework.
4. Monopolistic Competition: A more common real-world model with many firms, differentiated products, and free entry and exit. This sub-topic adapts the core profit maximization rule from monopoly to account for free entry, which drives long-run economic profit to zero, and introduces the tradeoff between product variety and excess capacity.
5. Oligopoly and Game Theory: The most complex model, with a small number of interdependent firms, each of whose profit depends on the choices of other firms. This sub-topic introduces game theory, the tool for analyzing strategic interaction, and builds on all prior concepts of market power and profit maximization.

3. A Guided Tour: Multiple Sub-Topics In Action

To see how all sub-topics connect in a single exam-style problem, consider the following scenario: "The prescription drug market for a new diabetes treatment has only one firm that holds a 20-year patent on the drug, giving it exclusive right to sell the drug. The firm wants to charge different prices for the drug in the US vs in low-income countries."

Let's walk through how we apply sub-topics in sequence to analyze this problem:

1. Step 1: Classify the market (Introduction to Imperfectly Competitive Markets): First, we confirm this is an imperfectly competitive market: the firm has market power from being the only seller, so it faces a downward-sloping demand curve, and marginal revenue is less than price. This immediately rules out a perfectly competitive analysis.
2. Step 2: Find the single-price monopoly outcome (Monopoly): We use the core monopoly procedure: calculate marginal revenue from the market demand curve, set $MR=MC$ to find the profit-maximizing quantity, then set price from the demand curve. We can also calculate the deadweight loss from this monopoly outcome, compared to the socially efficient quantity where $P=MC$.
3. Step 3: Analyze the firm's pricing strategy (Price Discrimination): The firm's plan to charge different prices in different countries is third-degree price discrimination, which builds on our monopoly profit analysis. We can show that this strategy increases the firm's total profit, and can also increase total social surplus by making the drug available to low-income consumers who would not have been able to buy it at the higher US-only price.
4. Step 4: What happens when the patent expires? (Monopolistic Competition): When the patent expires, dozens of generic drug makers enter the market, each selling a slightly differentiated version of the drug, with no barriers to further entry. We now apply the monopolistic competition framework: entry shifts each firm's demand curve inward until all firms earn zero economic profit in the long run.

This guided tour shows how each sub-topic builds on the prior one to answer a single, real-world question, starting from foundational classification to specific outcome analysis.

4. Common Cross-Cutting Pitfalls (and How To Avoid Them)

• Wrong move: Treating marginal revenue as equal to price for any imperfectly competitive firm. Why: Students memorize $MR=P$ from perfect competition and incorrectly carry this rule over to all market structures. This leads to wrong profit-maximizing quantity and price calculations. Correct move: For any firm with a downward-sloping demand curve (all imperfectly competitive firms), always draw marginal revenue below the demand curve, and use $MR=MC$ (not $P=MC$) to find the profit-maximizing quantity.
• Wrong move: Assuming all imperfectly competitive markets earn positive long-run economic profit. Why: Students learn that monopolies earn positive long-run profit due to barriers to entry, and incorrectly generalize this to all imperfect competition structures. Correct move: Explicitly check for barriers to entry when predicting long-run profit: free entry in monopolistic competition drives long-run economic profit to zero, just like perfect competition; only markets with persistent barriers to entry (monopoly, collusive oligopoly) have positive long-run profit.
• Wrong move: Claiming all price discrimination always reduces total social surplus. Why: Students associate all market power with deadweight loss, so they generalize that any use of price discrimination makes society worse off. Correct move: Analyze welfare effects of price discrimination on a case-by-case basis: perfect price discrimination eliminates deadweight loss (though all surplus goes to the firm), while third-degree price discrimination can increase or decrease total surplus depending on how it changes total output.
• Wrong move: Forcing a dominant strategy and incorrect Nash equilibrium in games that do not have dominant strategies. Why: Most introductory examples are prisoner's dilemmas that each have a dominant strategy, so students assume all games follow this pattern. Correct move: Always follow the standard procedure for solving games: first check for a dominant strategy for each player by comparing payoffs across all opponent choices; if no dominant strategy exists, find the Nash equilibrium by identifying mutual best responses.
• Wrong move: Confusing zero long-run economic profit for monopolistic competition with zero accounting profit, and assuming the firm will exit the market. Why: Students mix up economic and accounting profit definitions from earlier in the course, leading to wrong conclusions about firm entry/exit. Correct move: Whenever you see "zero economic profit", explicitly recall that this means the firm is earning a normal rate of return equal to its opportunity cost, so it has no incentive to enter or exit the market.

5. Quick Check: Do You Know When To Use Which Sub-Topic?

For each scenario below, identify which of the 5 unit sub-topics you would use to analyze it:

1. A small town has only one hospital. What price will it charge, and how much deadweight loss will it create?
2. 15 hair salons all offer slightly different services in a suburban area, with no barriers to opening new salons. What will happen to the average salon's profit in the long run?
3. Two large automakers are deciding whether to spend millions on a new Super Bowl ad campaign. What outcome will we see?
4. A coffee shop offers a 10% discount to students with a valid school ID. How does this discount affect the coffee shop's total profit?
5. What core feature do all markets where firms can set their own price share, compared to perfectly competitive markets?

Solutions:

1. Monopoly
2. Monopolistic Competition
3. Oligopoly and Game Theory
4. Price Discrimination
5. Introduction to Imperfectly Competitive Markets

If you got all 5 correct, you are ready to dive into the detailed sub-topics; if you missed any, review the concept map above to refresh how each sub-topic is defined.

6. Practice Questions (AP Microeconomics Style)

Question 1 (Multiple Choice)

Which of the following statements is true for both single-price monopoly and monopolistic competition in long-run equilibrium? A) Both produce the socially efficient quantity where $P=MC$ B) Both earn zero economic profit in the long run C) Both face downward-sloping demand curves with $MR<P$ D) Both have free entry and exit of firms

Worked Solution: Eliminate each option one by one. Option A is incorrect: both have market power, so profit maximization occurs where $MR=MC$, and since $MR<P$, this means $P>MC$, which is inefficient. Option B is incorrect: only monopolistic competition has zero long-run economic profit due to free entry; monopolies are protected by barriers to entry, so they earn positive long-run economic profit. Option D is incorrect: free entry and exit is only a feature of monopolistic competition, not monopoly. Option C is correct: both are imperfectly competitive firms with market power, so both face downward-sloping demand and marginal revenue is less than price. The correct answer is C.

Question 2 (Free Response)

A single movie theater is the only theater in a small rural town. It faces the demand function $P=18-Q$, and has constant marginal cost of $3$ per customer. (a) Derive the theater's marginal revenue function. Calculate the profit-maximizing quantity and price for the theater as a single-price monopolist. (b) The theater decides to charge a lower price for students than for non-students, based on showing a student ID. What type of price discrimination is this? Will the theater's total profit be higher or lower than it was in part (a)? Explain. (c) If two new movie theaters open in the town, each showing slightly different selections of movies, with no additional barriers to entry, what will happen to the long-run economic profit of each theater compared to the original monopoly? Name the new market structure.

Worked Solution: (a) Total revenue $TR=P\times Q=(18-Q)Q=18Q-Q^2$. Marginal revenue is the derivative of total revenue, so $MR=18-2Q$. Set $MR=MC$ to find profit-maximizing quantity: $18-2Q=3\to 2Q=15\to Q=7.5$. Plug back into demand to find price: $P=18-7.5=\$10.50$. (b) This is third-degree price discrimination, which separates customers into two groups with different price elasticities of demand (students have more elastic demand than non-students). By charging different prices to each group, the theater captures more consumer surplus than it can with a single uniform price, so total profit will be higher than in part (a). (c) Free entry of new firms means the market becomes monopolistically competitive. Entry will shift each firm's demand curve inward as customers split between more firms, until long-run economic profit for each firm falls to zero, which is lower than the original monopoly's positive economic profit. The new market structure is monopolistic competition.

Question 3 (Real-World Application)

The US soft drink market is dominated by two large firms: Coca-Cola and PepsiCo, which together control roughly 70% of the market, with significant barriers to entry from brand recognition and distribution networks. Both firms spend hundreds of millions of dollars annually on advertising to gain market share from each other. Identify the market structure of the US soft drink market, name the framework you would use to analyze the firms' advertising decisions, and explain why this framework is appropriate.

Worked Solution: The US soft drink market has two large dominant firms, high barriers to entry, and each firm's profit depends directly on the advertising decision of the other firm, so this market structure is an oligopoly. The appropriate framework to analyze their interdependent advertising decisions is game theory, which is the standard tool for studying strategic interaction between firms in oligopoly. Game theory is appropriate here because each firm's choice to advertise heavily or not affects the profit of the other firm, so each firm must consider the other's likely response when making its own decision. In most cases, this game results in a prisoner's dilemma where both firms advertise more than is jointly optimal, but neither can cut advertising without losing market share to the other.

7. Unit Quick Reference Cheatsheet

8. See Also (Detailed Sub-Topic Study Guides)

• Introduction to Imperfectly Competitive Markets
• Monopoly
• Price Discrimination
• Monopolistic Competition
• Oligopoly and Game Theory