---
title: "Herfindahl-Hirschman Index"
type: "entity"
entity_kind: "concept"
status: "hub"
canonical_name: "Herfindahl-Hirschman Index"
aliases: ["HHI","Herfindahl index"]
first_seen: "2026-10-31T00:00:00.000Z"
writer_model: "claude-sonnet-5"
connects_to: ["media concentration","antitrust","Eli Noam","news ownership","owner-level vs outlet-level measurement"]
seek_code_commit: "d40ee39"
---


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]]