Introduction to Imperfectly Competitive Markets — AP Microeconomics
1. Core Definition of Imperfect Competition ★★☆☆☆ ⏱ 3 min
Imperfectly competitive markets are any market structure that deviates from the strict assumptions of perfect competition. At least one firm has **market power**: the ability to set price above marginal cost, rather than taking the market price as given.
This foundational topic for AP Microeconomics Unit 4 (which counts for 16-22% of total AP exam score) introduces core tools that apply to all three main imperfect competition structures: monopoly, monopolistic competition, and oligopoly. These tools are also used for antitrust analysis of market power.
2. Downward-Sloping Demand and Marginal Revenue ★★★☆☆ ⏱ 4 min
Unlike perfectly competitive firms, which face horizontal (perfectly elastic) demand at the market price, all imperfectly competitive firms face a downward-sloping demand curve. To sell additional output, a firm with market power must lower its price for *all units sold*, not just the extra unit. This means marginal revenue (additional revenue from selling one more unit) is always less than price, or $MR < P$ for all $Q>0$.
For a linear inverse demand curve of the form $P = a - bQ$, we derive marginal revenue by first calculating total revenue, then finding its derivative:
TR = P \times Q = (a - bQ)Q = aQ - bQ^2 \\ MR = \frac{dTR}{dQ} = a - 2bQ
For linear demand, MR has the same vertical intercept as the inverse demand curve but *twice the slope*, so it is always below the demand curve and crosses the horizontal axis at half the quantity where demand crosses the horizontal axis. We can also relate MR to price elasticity of demand:
MR = P\left(1 - \frac{1}{|E_d|}\right)
This relationship means: $MR>0$ when demand is elastic ($|E_d|>1$), $MR=0$ when demand is unit elastic ($|E_d|=1$), and $MR<0$ when demand is inelastic ($|E_d|<1$).
Exam tip: On AP graph questions, always draw MR below the demand curve for any imperfectly competitive firm. If you draw MR on the same line as demand, you will lose points even if your profit-maximizing quantity is correct.
3. The Lerner Index of Market Power ★★★☆☆ ⏱ 3 min
Market power is formally defined as a firm’s ability to set price above marginal cost. The Lerner Index is a standardized measure of the degree of market power a firm has, derived directly from the profit-maximizing condition $MR=MC$.
P\left(1 - \frac{1}{|E_d|}\right) = MC \\ L = \frac{P - MC}{P} = \frac{1}{|E_d|}
The Lerner Index ranges from 0 (no market power, $P=MC$, which describes perfect competition) to 1 (maximum market power, where $|E_d|=1$ and $MC$ approaches 0). The more inelastic the firm’s demand, the higher the Lerner Index, and the greater the firm’s market power. Barriers to entry allow firms to maintain market power in the long run.
Exam tip: Always remember the Lerner Index only applies to profit-maximizing firms where $MR=MC$ holds. If a question asks for the Lerner Index at a non-profit-maximizing quantity, you cannot use the $1/|E_d|$ shortcut—you have to calculate $(P-MC)/P$ directly.
4. Market Concentration Measures ★★★☆☆ ⏱ 4 min
To measure the degree of market power across an entire market (rather than just one firm), two common concentration measures are regularly tested on the AP exam: the 4-firm concentration ratio (CR4) and the Herfindahl-Hirschman Index (HHI).
Markets with HHI above 2500 are classified as highly concentrated by U.S. antitrust regulators.
Exam tip: When calculating HHI, always use market shares in percentage points (e.g., 45 for 45%, not 0.45) to get the standard 0-10000 range AP expects. Using decimals will give you a result 10,000 times too small and cost you points.
Common Pitfalls
Why: Students confuse the perfect competition case (where $MR=P=$demand) with imperfect competition, carrying over the wrong graph convention
Why: Students forget to multiply price by quantity to get total revenue before taking the derivative
Why: Students group all non-perfect competition into one category, ignoring differences in barriers to entry
Why: Students confuse the reason for MR < P with properties of cost curves
Why: Confusion over the standard scaling of HHI for the AP exam