Herfindahl-Hirschman Index

The standard antitrust concentration measure: the sum of the squared market shares of all firms in a market, used by US regulators (adopted by the FCC in 2005, per Carragher & Carley 2026) to judge whether a market is concentrated enough to warrant scrutiny. It matters to this vault as the shared arithmetic underneath its first media-concentration cluster: whether a news market's HHI is computed over individual outlets or over the companies that own them changes the measured answer, and by how much is itself a load-bearing finding (see the owner-level note below) rather than a settled constant.

References

• claim-owner-level-news-concentration-130x-outlet-level
• claim-ai-citation-concentration-exceeds-traffic-concentration