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Herfindahl Hirschman Index

Market concentration is a key concept in economics, and understanding it is crucial for businesses, policymakers, and regulators. One tool that helps in assessing market concentration is the Herfindahl-Hirschman Index (HHI). In this blog post, we will dive deep into the HHI and explore its significance in analyzing market structure, assessing competitiveness, and making informed business decisions.

The Herfindahl-Hirschman Index, commonly known as the HHI, is a measure of market concentration that takes into account the market shares of all firms in a specific industry. By calculating the HHI, we can determine the level of competition or concentration within an industry. A higher HHI indicates a more concentrated market with fewer competitors, while a lower HHI suggests a more competitive market with a larger number of firms. In the telecommunications industry, for example, a high HHI may indicate limited competition and potential market power held by a few major players. On the other hand, industries with a low HHI, such as the smartphone app market, often exhibit a higher level of competition with numerous small and medium-sized firms vying for market share.

It is calculated by summing the squares of the market shares of all firms operating in the market.

The formula for calculating the HHI is as follows:



  • HHIHHI is the Herfindahl-Hirschman Index.

  • nn is the number of firms in the market.

  • sisi is the market share of the ithith firm.

The resulting HHI value ranges from 0 to 10,000, where a higher HHI indicates a higher level of market concentration.

Here's how the HHI reflects the distribution of market shares among firms:

  1. Low Concentration (Competitive Market):If the HHI is low (close to 0), it suggests that the market is highly competitive, with a large number of firms having relatively equal market shares.

  2. Moderate Concentration:As the HHI increases, it indicates a moderate level of concentration. This might suggest that a few larger firms dominate the market, but there is still a significant presence of smaller competitors.

  3. High Concentration (Oligopoly or Monopoly):A very high HHI (approaching 10,000) signifies a highly concentrated market. In such cases, a small number of firms control a substantial portion of the market, and there may be concerns about reduced competition, potentially leading to anti-competitive behavior.

  4. Changes Over Time:Changes in the HHI over time can indicate shifts in market structure. For example, an increase in HHI over time may signal a trend toward increased market concentration, while a decrease may suggest a more competitive environment.

Case Studies in Market Power

The Herfindahl-Hirschman Index (HHI) has been utilized to analyze various industries and provide insights into market power and competition. Through the lens of the HHI, case studies can be conducted to uncover trends, shifts in market dynamics, and the impact of mergers and acquisitions on competition.

  1. Telecommunications Industry: AT&T and Time Warner Merger (2018)

  • In 2011, AT&T announced its intention to acquire T-Mobile USA, a move that would significantly impact the U.S. wireless telecommunications market.

  • The Department of Justice (DOJ) and the Federal Communications Commission (FCC) used the HHI to assess the potential antitrust implications of the merger.

  • The HHI calculations revealed that the combined market share of AT&T and T-Mobile would result in a substantial increase in market concentration, leading to reduced competition.

  • As a result, the DOJ filed a lawsuit to block the merger, arguing that it would harm consumers by limiting choices and potentially raising prices. Eventually, AT&T abandoned the acquisition.

  1. Pharmaceutical Industry: Pfizer and Allergan Merger (2016)In 2009, Pfizer acquired Wyeth in a multi-billion dollar deal, consolidating two major pharmaceutical companies.Regulatory authorities, including the Federal Trade Commission (FTC), used the HHI to evaluate the impact of the merger on the pharmaceutical industry's competitive landscape.The HHI analysis helped identify markets where the concentration would be significantly increased, potentially leading to reduced competition and higher drug prices.The FTC required Pfizer to divest certain products to address antitrust concerns and maintain a competitive market.

  2. Airlines Industry: American Airlines and US Airways Merger (2013)In 2013, American Airlines and US Airways announced their intention to merge, creating the world's largest airline by passenger traffic.Regulatory bodies, including the Department of Justice (DOJ) and state attorneys general, scrutinized the merger to assess its potential impact on competition within the airline industry.The HHI was a key tool used to measure market concentration and evaluate the merger's effects on competition. Calculations involved examining the market shares of both airlines across various routes and hubs.The analysis revealed that the merger would result in increased concentration, particularly in certain airport hubs where both American Airlines and US Airways operated.Antitrust concerns centered around the possibility of reduced competition, higher airfares, and diminished service quality for consumers in markets where the two airlines overlapped significantly.To address these concerns, the DOJ, along with several states, reached a settlement with American Airlines and US Airways. The settlement required the divestiture of slots and gates at key airports, including Washington National, New York LaGuardia, Chicago O'Hare, Boston Logan, Los Angeles International, and others.By divesting these assets, the DOJ aimed to preserve competition and allow other carriers to enter or expand in these markets, mitigating the potential negative effects of the merger on consumers.

  3. Healthcare Industry: CVS Health and Aetna Merger (2018)In 2018, CVS Health, a leading pharmacy and retail healthcare company, announced its acquisition of Aetna, one of the largest health insurance providers in the United States.The merger aimed to create an integrated healthcare company that could offer a broad range of services, including pharmacy benefits, retail healthcare, and insurance coverage.Regulatory scrutiny, particularly from the Department of Justice (DOJ) and state attorneys general, focused on potential antitrust concerns and the impact of the merger on competition in various healthcare segments.The Herfindahl-Hirschman Index (HHI) was utilized to assess changes in market concentration within different healthcare services, including pharmacy benefit management and health insurance.HHI calculations demonstrated that the merger would lead to increased concentration in certain healthcare markets, raising concerns about potential anti-competitive effects.To address these concerns, the DOJ required CVS Health and Aetna to divest Aetna's Medicare Part D prescription drug plan business. The divestiture aimed to preserve competition in the market for Medicare Part D plans and mitigate potential adverse effects on consumers.The HHI played a crucial role in quantifying and illustrating the changes in market concentration that would result from the merger, assisting regulatory authorities in making informed decisions to protect competition in the healthcare industry.

  4. Technology Industry: Microsoft Antitrust Case (2001):

  • In the landmark Microsoft antitrust case, the HHI was used to evaluate the level of concentration in the operating systems market.

  • The calculation demonstrated that Microsoft held a dominant position in the market, leading to concerns about potential anti-competitive behavior.

  • The legal proceedings resulted in a settlement that imposed restrictions on Microsoft's business practices to foster greater competition and innovation.

Antitrust authorities often use the HHI to assess the potential impact of mergers and acquisitions on market competition. Higher concentrations might attract regulatory scrutiny, as they could lead to market power and reduced consumer welfare. By utilizing the HHI as a tool, businesses, policymakers, and regulators can gain valuable insights into the competitive landscape and make informed decisions to ensure fair competition and consumer welfare.

Using the Herfindahl-Hirschman Index for Business Strategy

The Herfindahl-Hirschman Index (HHI) is a valuable tool for businesses to analyze market concentration and competition. By assessing the HHI, businesses can:

  • Evaluate market entry opportunities by identifying industries with lower concentration levels, indicating a more competitive environment.

  • Understand market dynamics by monitoring changes in the HHI over time.

  • Strategically position themselves by comparing HHI values across different industries.

Incorporating the HHI into strategic decision-making processes can help businesses make informed choices, enhance competitiveness, and drive long-term success.

The Evolution of Market Competition: Tracing Trends with the Herfindahl-Hirschman Index

The HHI also allows us to track changes in market competition over time. By comparing HHI values from different periods, we can identify trends and shifts in market structure. For instance, a steadily increasing HHI might indicate a trend towards market consolidation and decreased competition, while a decreasing HHI might suggest a more dynamic and competitive market environment.

Implications for Business Strategy:

  1. Market Power:High HHI values may signal the presence of market power among a few dominant firms. In such cases, these firms might have the ability to influence prices, control supply, and impact overall market dynamics.

  2. Strategic Planning:Businesses can use the HHI to inform their strategic planning. In concentrated markets, firms may focus on differentiation, cost leadership, or niche strategies to gain a competitive edge.

  3. Entry and Exit Strategies:Lower HHI values might encourage new entrants, as a less concentrated market is typically more accessible. Conversely, high concentration may discourage new entrants, prompting businesses to consider alternative markets or strategies.

  4. Regulatory Compliance:Regulatory authorities often use the HHI to assess the competitive impact of mergers and acquisitions. A significant increase in market concentration may raise antitrust concerns, leading to regulatory scrutiny.

  5. Risk Management:Businesses can assess the level of risk associated with market concentration. In highly concentrated markets, firms may be more susceptible to economic downturns, changes in consumer preferences, or disruptions caused by a dominant player.

Consider an industry with three companies, each holding 40%, 30%, and 30% market share, respectively. The HHI would be calculated as follows:

HHI = (0.4^2 + 0.3^2 + 0.3^2) = 0.16 + 0.09 + 0.09 = 0.34

This relatively low HHI (0.34) suggests a moderately competitive market with a balanced distribution of market share among three firms.

Calculation and Methodology


where sisi represents the market share of firm ii in the market and nn is the number of firms in the market.

Here's a breakdown of the calculation and its components:

  1. Market Share (sisi): The market share of each firm is expressed as a percentage of the total market. For example, if there are three firms in a market and their respective market shares are 30%, 40%, and 30%, then s1=0.3s1=0.3, s2=0.4s2=0.4, and s3=0.3s3=0.3.

  2. Squaring (si2si2): Each firm's market share is squared. This squaring penalizes larger market shares more than smaller ones. The idea is to give more weight to the larger firms, reflecting their greater influence on market concentration.

  3. Summation (∑i=1nsi2∑i=1*nsi*2):** The squared market shares of all firms are summed to obtain the HHI for that particular market. The HHI ranges from a minimum of 0, indicating perfect competition (where all firms have equal market shares), to a maximum of 10,000, representing a monopoly (where one firm dominates the entire market).

Limitations of HHI:

  1. Scale Sensitivity: The HHI is sensitive to the scale of measurement. For instance, if market shares are expressed in percentages, the index will be different from when they are expressed as fractions. This makes it essential to ensure consistency in units when comparing HHI values across different markets.

  2. Ignores Market Dynamics: HHI is a static measure and doesn't consider changes over time. It doesn't account for factors such as entry and exit of firms, mergers and acquisitions, or changes in market conditions.

  3. Assumes Equal Competitiveness: HHI assumes that all firms are equally competitive, regardless of their size. In reality, larger firms may have different levels of market power, and HHI may not fully capture these nuances.

Alternative Measures:

  1. Concentration Ratios: These ratios provide a simpler measure of market concentration by summing the market shares of the top 'X' firms. For example, a 4-firm concentration ratio adds the market shares of the four largest firms in the market.

  2. Gini Coefficient: Originally used to measure income inequality, the Gini coefficient has been adapted to assess market concentration. It considers the distribution of market shares and provides a more nuanced view of concentration.

  3. Entropy Index: This index considers the distribution of market shares but gives more weight to smaller firms. It offers a different perspective on market concentration, especially when there are many small firms in the market.

While the Herfindahl-Hirschman Index is a widely used measure of market concentration, it's crucial to consider its limitations and complement it with other measures to gain a more comprehensive understanding of the competitive dynamics within a market.

Evolving Market Dynamics

Examining historical trends in the Herfindahl-Hirschman Index (HHI) provides insights into market concentration's impact on economic growth, innovation, and consumer welfare. The HHI fluctuates as industries progress from low concentration and high innovation to consolidation and potential challenges to innovation. Low concentration fosters innovation and entrepreneurship, while consolidation can lead to efficiency gains but may reduce incentives for innovation. Competitive markets drive innovation, while high concentration may stifle it. Consumer welfare benefits from variety and competitive pricing in low concentration markets, but quality and pricing depend on dominant firms' behavior in highly concentrated markets. Policymakers can implement antitrust measures and incentives for innovation to maintain a balance between consolidation and a competitive market.


In conclusion, the Herfindahl-Hirschman Index (HHI) serves as a valuable tool for understanding market concentration and competition. By calculating the HHI, businesses, policymakers, and regulators gain valuable insights into the dynamics of various industries. The HHI helps identify the level of competition or concentration within a market, guiding strategic decisions, assessing antitrust implications, and promoting fair competition. Understanding the implications of market concentration and tracking trends over time enables stakeholders to make informed choices that foster innovation, drive economic growth, and prioritize consumer welfare. With its ability to shed light on market structure and dynamics, the HHI plays a crucial role in navigating the ever-changing economic landscape.

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