AP Microeconomics Unit 4: Imperfect Competition Notes

Introduction

Imperfectly competitive firms choose output where marginal revenue meets marginal cost and then read price from demand, but the long-run result depends on barriers to entry, product differentiation, and strategic responses.

Market structure is a set of assumptions, not a label based only on the number of familiar brands. The relevant evidence includes product substitutability, ease of entry, the firm’s demand curve, scale economies, and whether each seller must anticipate competitors.

A downward-sloping demand curve makes marginal revenue lower than price under single-price selling. Monopoly, price discrimination, monopolistic competition, and oligopoly then modify the output, surplus, entry, and strategy analysis in different ways. Efficiency results remain conditional and do not by themselves determine legal or policy conclusions.

Entry conditions and substitutes

market structure

Downward firm demand

MR below price

MR meets MC

chosen quantity

Chosen quantity on demand

price

Segmentation and resale barriers

price discrimination

Entry and differentiated products

long-run tangency

Rival best responses

Nash equilibrium

Current exam guidance: Aligned to the current 2026–27 AP Microeconomics framework. Every model identifies axes, assumptions, movement or shift, market structure, and the quantitative or welfare consequence.

4.1

Introduction to Imperfectly Competitive Markets

Which assumptions distinguish monopoly, monopolistic competition, oligopoly, and perfect competition?

Short answer

Market structure depends on entry barriers, product substitutability, pricing discretion, and strategic interdependence—not on seller count by itself.

Know firstPerfect competition · Substitutes · Economies of scale

Classify the structure from behavior and constraints

A perfectly competitive seller is a price taker because buyers regard its product as identical to alternatives and entry is relatively easy. A monopolistically competitive firm has many rivals but sells a differentiated product, giving it some downward-sloping demand. An oligopoly has few strategically important sellers, so each firm’s best action depends on expected rival responses. A monopoly has no close competing seller in the defined market and faces substantial entry barriers.

Market power means the ability to influence price through output or product choice. It can arise from product differentiation, control of an essential input, legal protection, network effects, or scale economies. None of these mechanisms automatically proves harmful or illegal conduct. A market with 30 differentiated sellers is not perfectly competitive merely because it has “many” firms, and a concentrated market cannot be classified without defining the product and geographic market and examining entry.

Economic model

Use entry and pricing discretion as separate dimensions

Place four synthetic markets using evidence about entry and product substitution instead of relying on seller count alone.

Held constant: The four cases are synthetic and the market boundary has already been defined. The plotted indices summarize the accompanying qualitative evidence.
Time horizon: Current market conditions, with entry evaluated over a horizon long enough for a potential competitor to establish production.
Read the change: This is a classification map. A durable change in product substitutability or entry conditions can move a market to a different region.

Dimensions shown: entry and pricing discretion

Synthetic market cases

Horizontal axis: Entry barrier index
Vertical axis: Pricing discretion index

1many sellers, homogeneous product, easy entry230 differentiated sellers, easy entry3few interdependent sellers, difficult entry4one seller without a close substitute, durable barrier

The indices organize evidence; they are not legal thresholds

\text{structure}=f(\text{substitutes, entry, scale, strategic interaction})

Four anonymous cases move from easy entry and little pricing discretion toward difficult entry and substantial discretion. A 30-seller differentiated market is separated from the homogeneous competitive case.

Read the model as a table

Use entry and pricing discretion as separate dimensions values
Case	Sellers	Product relation	Entry	Strategic interaction	Classification
A	many	homogeneous	easy	minimal	perfect competition
B	30	differentiated	relatively easy	limited	monopolistic competition
C	few	close substitutes	difficult	strong	oligopoly
D	one	no close substitute	durable barrier	not applicable	monopoly

Classify without using a shortcut

Define the product and geographic market.
Assess whether products are homogeneous or differentiated.
Identify legal, technological, network, input, or strategic entry barriers.
Ask whether each seller treats rival reactions as a central constraint.
Use seller count only after those conditions are known.

Keep these three ideas

Market structure is defined by more than seller count.
Product differentiation creates some pricing discretion.
Oligopoly adds strategic interdependence.

Imperfect Competition