Difficulty: Introductory to Intermediate | Prerequisites: Supply, Demand, and Market Equilibrium notes (Part 1)
This set covers three related clusters: the theory of trade (absolute and comparative advantage), how economists measure market concentration (CR4 and HHI), and how product differentiation shapes competition. These topics are tested through definitions, short calculations, and comparison questions. They connect directly to the market structures covered in Part 2.
Absolute advantage is about who produces more with the same resources; comparative advantage is about who produces at a lower opportunity cost, and it is comparative advantage that drives the gains from trade. Market concentration is measured by CR4 (top four firms' combined share) and HHI (sum of squared shares). Products can be differentiated vertically (by quality everyone agrees on) or horizontally (by taste that varies across consumers).
Absolute advantage
The ability to produce more of a good using the same amount of resources as another producer. Think of it as raw productivity: who is faster or more efficient in absolute terms.
Comparative advantage
The ability to produce a good at a lower opportunity cost than another producer. This is the concept that explains why trade is beneficial, even when one party is better at producing everything. In simple terms, it is about what you give up, not how much you make.
4-firm concentration ratio (CR4)
The sum of the market shares of the four largest firms in an industry, expressed as a percentage. A CR4 of 90% means the top four firms control 90% of the market.
Herfindahl-Hirschman Index (HHI)
A measure of market concentration calculated by squaring the market share of each firm in the industry and summing the results. HHI ranges from close to 0 (many tiny firms) to 10,000 (a single firm with 100% share). The US Department of Justice considers an HHI above 2,500 to indicate a highly concentrated market.
Horizontal product differentiation
Products differ in characteristics that are a matter of personal taste. At the same price, different consumers would choose different products. Think of it as flavour preference: some people prefer Coke, others prefer Pepsi, and neither choice is objectively "better."
Vertical product differentiation
Products differ in quality in a way that all consumers agree on. If prices were equal, everyone would choose the higher-quality product. Think of it as a clear ranking: a fast processor is better than a slow one by any measure.
Absolute advantage compares productivity levels directly. If Country A can produce 10 units of wheat per hour and Country B can produce 6, Country A has the absolute advantage in wheat.
Comparative advantage compares opportunity costs. If Country A gives up 2 units of cloth to produce 1 unit of wheat, and Country B gives up only 1 unit of cloth per wheat, then Country B has the comparative advantage in wheat (lower opportunity cost).
A country (or firm) can have an absolute advantage in everything, but it cannot have a comparative advantage in everything. This is mathematically impossible.
Trade is beneficial when each party specialises in the good where it has a comparative advantage. Both sides end up with more than they could produce alone.
CR4 (4-firm concentration ratio)
Add the market shares of the four largest firms.
Simple to calculate but tells you nothing about how those shares are distributed among the four, or about the remaining firms.
HHI (Herfindahl-Hirschman Index)
Square each firm's market share (as a whole number, e.g. 35 not 0.35) and sum all the squares.
Gives more weight to firms with large shares, so it captures the unevenness of market power better than CR4.
HHI interpretation: below 1,500 is unconcentrated, 1,500 to 2,500 is moderately concentrated, above 2,500 is highly concentrated.
When a group of small firms is reported as a block (e.g. "Others: 10%"), you cannot square the block as one firm. If you do not know the individual shares, the convention is to treat them as very small firms (e.g. ten firms at 1% each), which contributes very little to the HHI.
Vertical differentiation
Quality ranking that everyone agrees on.
Example: a high-speed processor vs a slow one, a luxury hotel vs a budget hotel.
At the same price, all consumers pick the higher-quality option.
Price differences persist because consumers differ in willingness to pay for quality.
Horizontal differentiation
Variety based on personal taste.
Example: Coke vs Pepsi, vanilla vs chocolate ice cream, red vs blue.
At the same price, consumers split across options based on preference.
Firms can charge above marginal cost because loyal customers value one variety over another.
CR4 calculation
CR4 = s_1 + s_2 + s_3 + s_4where s_1 through s_4 are the market shares of the four largest firms.
Worked example (Beryllium oxide industry): Mercury 35%, Mars 25%, Jupiter 20%, Saturn 10%, Others 10%.
CR4 = 35 + 25 + 20 + 10 = 90%
HHI calculation
HHI = \sum_{i=1}^{N} s_i^2where s_i is each firm's market share (as a whole number, not a decimal).
Worked example (same industry):
35^2 + 25^2 + 20^2 + 10^2 = 1,225 + 625 + 400 + 100 = 2,350
The "Others" block at 10% is composed of multiple small firms. If treated as ten firms at 1% each, they add 10 x 1 = 10, giving HHI = 2,360. If treated as one block of 10%, they add 100, giving HHI = 2,450.
Either approach (2,350 to 2,450) earns full marks on this exam. The key is showing the squaring method.
Comparative advantage explains why countries trade even when one is more productive across the board. The US and Bangladesh both benefit from trade in textiles and aircraft because each specialises where its opportunity cost is lower.
The HHI is used by antitrust regulators (the US Department of Justice, the FTC) to evaluate proposed mergers. A merger that pushes HHI above 2,500, or increases it by more than 200 points, triggers closer scrutiny.
Students often confuse absolute and comparative advantage. A country with an absolute advantage in both goods still has a comparative advantage in only one of them. The one with the lower opportunity cost is the one it should specialise in.
Students sometimes square the "Others" block as one firm when calculating HHI. The correct approach treats it as many small firms, each contributing a tiny squared share. In practice, this means the "Others" block adds very little to the HHI.
Students often mix up horizontal and vertical differentiation. The test is simple: if prices were identical, would all consumers agree on which product is better? If yes, it is vertical. If consumers would split, it is horizontal.
⚠️ A definition question on comparative vs absolute advantage is almost certain. Know both definitions cold, and know that a firm can have an absolute advantage in everything but cannot have a comparative advantage in everything.
⚠️ The CR4 and HHI calculation question uses the Beryllium oxide data. Show your working: list each firm's share, square each one, and sum.
⚠️ Horizontal vs vertical differentiation appears as both a multiple-choice question and a short-answer question asking for examples. Have one example ready for each.
⚠️ The HHI question may ask for interpretation (is this market concentrated?). Know the thresholds: under 1,500, 1,500 to 2,500, over 2,500.
True or false: A country with an absolute advantage in both goods will also have a comparative advantage in both. (False. Comparative advantage can only be held in one of the two goods.)
Fill in the blank: The HHI is calculated by ________ each firm's market share and ________ the results. (Squaring; summing.)
True or false: Vertical product differentiation means different consumers prefer different products at the same price. (False. That describes horizontal differentiation. With vertical differentiation, all consumers agree on the ranking.)
Fill in the blank: CR4 for an industry where the top four firms hold 35%, 25%, 20%, and 10% is ________. (90%.)
Q: Define comparative advantage and explain how it differs from absolute advantage.
A: Absolute advantage is the ability to produce more of a good using the same resources. Comparative advantage is the ability to produce a good at a lower opportunity cost. A country can have an absolute advantage in everything, but it will still benefit from trade by specialising where its opportunity cost is lowest. You cannot have a comparative advantage in everything.
Q: Calculate the CR4 and HHI for the Beryllium oxide industry (Mercury 35%, Mars 25%, Jupiter 20%, Saturn 10%, Others 10%).
A: CR4 = 35 + 25 + 20 + 10 = 90%. HHI = 35^2 + 25^2 + 20^2 + 10^2 = 1,225 + 625 + 400 + 100 = 2,350 (treating Others as many small firms). This indicates a highly concentrated market.
Q: Describe the difference between horizontal and vertical product differentiation, with one example of each.
A: Vertical differentiation: products differ in quality, and all consumers agree on the ranking. Example: a high-speed processor vs a slow one. At equal prices, everyone picks the faster one. Horizontal differentiation: products differ in features that are a matter of taste. Example: Coke vs Pepsi. At equal prices, different consumers choose differently based on flavour preference.
Comparative advantage and opportunity cost (Part 1) are the same concept applied to trade. Understanding why the low-opportunity-cost producer should specialise requires fluency with the opportunity cost definition.
HHI and CR4 connect to the market structures in Part 2: a high HHI suggests the market is close to monopoly, while a low HHI suggests something closer to perfect competition. Product differentiation determines whether Bertrand competition drives price to MC (homogeneous) or allows firms to charge above MC (differentiated).
Absolute advantage, comparative advantage, gains from trade, specialisation, opportunity cost, CR4, four-firm concentration ratio, HHI, Herfindahl-Hirschman Index, market concentration, horizontal differentiation, vertical differentiation, product differentiation, quality ranking, taste-based preferences, antitrust, merger analysis, ECO3101, FIU, intermediate microeconomics