Antitrust Radar
Methods and MeasuresAR-MM-2026-01

How HHI Helps Explain Market Concentration

A public primer for reading concentration dashboards

By Antitrust Radar Research TeamMethodology primerMay 16, 2026Version 1.0 · Current edition

Central finding

HHI is the sum of squared market shares.

Descriptive concentration measures do not by themselves establish competitive harm, unlawful conduct, or a causal effect.

3
sources
3
bounded examples
2 / 3
share-derived HHI

Publication accountability

Recorded source access / review
. This is not an independent review date.
Authorship and responsibility
Antitrust Radar Research Team. Publication custody and revision record.
Evidence overview

Reproducible measurement route

Compute when shares close; preserve the source when they do not.

Search decomposition

Audit seven shares against the published HHI

Open evidence

Browser decomposition

Inspect the complete public denominator

Open evidence

FCC evidence boundary

Keep the reported HHI without inferred shares

Open evidence
Sources, citations, and policy proposals

Public action docket

What government can require next

3 asks

ForAgency staff, journalists, and civic researchers

  1. 01
    Publish HHI with the source table

    When an agency cites concentration, release the firm-level shares or state clearly why only a source-reported HHI can be published.

  2. 02
    Label proxy markets plainly

    Usage-share, passenger, prescription, or revenue proxies should be named as proxies before readers infer legal conclusions.

  3. 03
    Show the uncertainty

    Pair HHI with CR4, top-firm share, confidence level, and a short note about missing firms or aggregated residual rows.

Evidence still needed

Firm-level share tableMarket geographyProxy-definition note
Open advocacy brief

Reader packet

Reusable

Canonical citation

Antitrust Radar Research Team. (2026, May 16). How HHI Helps Explain Market Concentration: A public primer for reading concentration dashboards. Methods and Measures, AR-MM-2026-01 (Version 1.0). Antitrust Radar. https://antitrustradar.org/reports/hhi-primer

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BibTeXRIS

Briefing packetcompute, audit, interpretHHI observations JSON3 bounded public readingsPublic share CSVexact rows where disclosedMethods standarddenominators and limitsFCC source dossieragency-reported HHI boundarySearch market dossierreproducible share decompositionShare-row APIparticipant-level evidenceAction briefturn measurement into oversight

Source basis

FCC source-reported HHI and StatCounter public usage-share tables.

3

Source records

3

Bounded examples

2

agency report, public dataset

Source types

4

Evidence asks

Read the briefing

Evidence index

Claim to public action

Published May 16, 2026 · sources through Jul 17, 2026 · version 1.0 · Current edition

Read / Briefing claim

Start with the authored claim and its analytical boundary.

Start with the share-squared decomposition, the source-reported boundary, and the controlled CR4 comparison.

Interpretation boundaryRead the finding with the report's declared market, geography, vintage, and limitations.
Read the analysis

Methods and Measures / AR-MM-2026-01

HHI reveals how shares are distributed, not why a market works.

Change the illustrative market, inspect two reproducible public share tables, and see where a source-reported HHI must remain undecomposed.
Formula
Σ share²
percentage shares / 0-10,000 scale
Public examples
3
one definition and period per reading
Share-derived
2
complete tables reconcile after rounding
Agency-reported
1
metric preserved / shares not inferred

Illustrative decomposition lab

Concentration rises when the same 100% becomes less evenly distributed.

Illustrative shares / not a real market
Illustrative HHI2,500
Effective firms4.0
CR4100%
Largest firm25%
625.0
Rival 125%
625.0
Rival 225%
625.0
Rival 325%
625.0
25² + 3 × 25.0²2,500

Public evidence examples

One formula, three publication paths.

Never compare without the denominator
Exact share decompositionΣ share² = 7359.0 → 7,359
100% share coverage
Google Search85.2%7252.2 HHI
Bing9.8%96.4 HHI
Yahoo Search2.7%7.1 HHI
DuckDuckGo1.7%3.0 HHI
Yandex0.4%0.1 HHI
Ecosia0.1%0.0 HHI
Other search engines0.1%0.0 HHI

CR4 and HHI

The same four-firm share can hide a different internal structure.

Controlled illustration / both CR4 = 80%
01

Balanced leaders

20% / 20% / 20% / 20% + 20 firms at 1%

CR4
80%
HHI
1,620
02

One dominant leader

50% / 10% / 10% / 10% + 20 firms at 1%

CR4
80%
HHI
2,820
1,200 HHI points apart

CR4 records only the combined top-four share. HHI also responds to how those four shares are distributed and to every smaller firm in the denominator.

Interpretation boundary

A reproducible number is still only one layer of competition evidence.

HHI summarizes the distribution of shares inside a specified product, geography, denominator, and period. It does not define the market, prove competitive harm, identify conduct, or repair missing firm-level shares. Antitrust Radar recomputes HHI only when one bounded public share table closes to 100 percent; otherwise it preserves the source-reported metric or withholds the calculation.
HHI can summarizeShare distribution

within one declared product, geography, denominator, and period.

HHI cannot establishMechanism or harm

including entry barriers, conduct, quality, prices, or competitive effects.

Publication ruleSource, compute, or withhold

Never manufacture missing firms or convert aggregate categories into individual shares.

Executive summary

HHI is the sum of squared market shares. It is useful because it gives larger firms more weight, but it is not a legal conclusion. This observatory reports HHI only when public sources provide enough reliable shares or when a public agency reports the metric directly. Market definition, geography, vintage, and missing firms can change interpretation. The mobile wireless profile uses FCC-reported HHI, while search and browser profiles use public usage-share tables with clear limitations.

Σ share²

HHI decomposition rule

Percentage shares are squared and summed, producing a zero-to-10,000 structural measure.

Illustrative decomposition lab

2 / 3

examples with public share tables

1 additional example preserves an agency-reported HHI without inventing missing firm shares.

Bounded public evidence

7,359

highest bounded public reading

The examples are deliberately not ranked against one another because their products, denominators, and periods differ.

Source-custodied examples

1,200

HHI-point gap at the same CR4

Two controlled structures both have CR4 of 80 percent, while HHI reveals a materially different internal distribution.

Controlled illustration

Research map

What this briefing connects

  • What HHI measures
  • Where CR4 helps
  • Why source notes matter

Source basis

FCC source-reported HHI and StatCounter public usage-share tables.

Bounded measurement evidence

Reuse the decomposition only when the public denominator is complete.

Search share rows

Reproduce the seven-category HHI

Open evidence

Browser share rows

Reproduce the complete public denominator

Open evidence

FCC source boundary

Preserve the HHI without inventing shares

Open evidence

Methodology and limits matter

HHI summarizes the distribution of shares inside a specified product, geography, denominator, and period. It does not define the market, prove competitive harm, identify conduct, or repair missing firm-level shares. Antitrust Radar recomputes HHI only when one bounded public share table closes to 100 percent; otherwise it preserves the source-reported metric or withholds the calculation.

Source basis

FCC source-reported HHI and StatCounter public usage-share tables.

Source register

3 cited public records

01 / 3
agency report2024

Source 01

2024 Communications Marketplace Report

Federal Communications Commission

Federal Communications Commission, 2024 Communications Marketplace Report, FCC 24-136, released December 31 2024.

Biennial FCC report on competition across communications markets including mobile wireless HHI.

Accessed
May 16, 2026
Custody
Public federal government report

Related reports

The U.S. Competition LandscapeFive Digital Gateways

Research actions

Audit HHI methodologyInspect share decompositionAudit FCC source boundary
Public action memoFoundation

What public institutions should do next

HHI should be a public-reading tool: useful for triage, but only credible when paired with market definition, source coverage, and caveats.

Audience

Agency staff, journalists, and civic researchers

1Publish HHI with the source tableWhen an agency cites concentration, release the firm-level shares or state clearly why only a source-reported HHI can be published.
2Label proxy markets plainlyUsage-share, passenger, prescription, or revenue proxies should be named as proxies before readers infer legal conclusions.
3Show the uncertaintyPair HHI with CR4, top-firm share, confidence level, and a short note about missing firms or aggregated residual rows.
Evidence needed
Firm-level share tableMarket geographyProxy-definition noteResidual category treatment
Connect to advocacy playbooks

Policy claims are framed as public-interest asks, not legal conclusions. Each ask should be read with the report’s source basis and limitations.