Market share¶
A supplier's or product's measured portion of a defined competitive market on a specified, compatible basis.
Core Idea¶
Market share is one supplier's or product's measured portion of a defined competitive market: its in-market amount divided by the matching market total. Market definition fixes product and geographic boundaries.[^ref-ea301e7803d7] The focal amount and total must share a measure and observation period or as-of date. US merger guidance considers revenue, units, capacity and non-price user measures according to market conditions[^ref-ea301e7803d7-2]; each measure yields a different share when paired with its compatible total: a Ratio specialized by a competitive-market boundary and focal participant.
Scope of Application¶
P&G's reported category shares[^ref-6e9fa2b4f16c] and merger analysis's product and geographic markets[^ref-ea301e7803d7] show where the aligned quotient travels when the focal amount and competitive total are compatible.
- Company categories: P&G tracks constant-currency Home Care dollar share and reported a 0.3-point rise using lagged estimates.[^ref-6e9fa2b4f16c]
- Merger assessment: the Novelis/Aleris decision compares automotive body-sheet sales shares by value and volume.[^ref-b22ded44006e]
- Capacity-sensitive markets: separate capacity shares can illuminate potential expansion beyond past sales.[ref-b22ded44006e][ref-ea301e7803d7-2]
- Low-price-exchange markets: user count or use frequency may be informative non-price measures.[^ref-ea301e7803d7-2]
- Buyer-side competition: an appropriate purchasing measure can replace sellers' product revenue.[^ref-ea301e7803d7-2]
Clarity¶
A firm's sales may rise while its share falls: sales of 10 out of 100 become 12 out of 150, moving share from 10% to 8%. The sharper question is whether the focal amount, market total, boundary, or measure changed; arithmetic alone does not identify a cause. A 0.3-percentage-point increase is a difference between share percentages, not a 0.3% sales increase or a disclosed share level.
Manages Complexity¶
A share compresses a focal amount and a matching market total into one relative-position figure. The compression helps compare participants and periods, provided the original basis stays visible. Value share can differ from unit share, and past sales can miss available capacity; the Novelis/Aleris assessment therefore examined capacity separately.[^ref-b22ded44006e] US guidance permits multiple metrics when market conditions make them informative.[^ref-ea301e7803d7-2] One percentage cannot carry all dimensions of competition.
Abstract Reasoning¶
First check the market boundary, focal participant, common measure, matched period or as-of date, and nonzero total. Then compare like with like. With positive totals in two periods, share rises exactly when focal_2 × total_1 > focal_1 × total_2; this derived test remains valid with a zero first-period focal amount. Historical shares may misstate future competitive significance.[^ref-ea301e7803d7-2] Treat an individual share as evidence of measured position, not a market-power verdict.[^ref-b22ded44006e]
Knowledge Transfer¶
The same aligned quotient works in a company category and an antitrust market even though those boundaries have different status. P&G reports Home Care value share.[^ref-6e9fa2b4f16c] The Commission examines separate value, volume and capacity shares.[^ref-b22ded44006e] These retain the focal-part and matching-total roles while answering different questions. Prime Ratio carries the portable division relation; a vote fraction or defect rate has that broader structure without becoming a market share. HHI squares and sums shares across firms.[^ref-2a71617345a3] Market power requires further evidence beyond an individual share.[^ref-ea301e7803d7-2]
Relationships to Other Abstractions¶
Current abstraction Market share Domain-specific
Parents (1) — more general patterns this builds on
-
Market share is a kind of Ratio Prime
Every market share is an aligned part-to-market-total ratio; it adds a competitive-market boundary and a supplier or product as the focal part.
Hierarchy path (1) — routes to 1 parentless root
- Market share → Ratio → Comparison
Neighborhood in Abstraction Space¶
Market share sits in a sparse region of the domain-specific corpus (93rd percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Unclustered & Miscellaneous (2721 abstractions)
Nearest neighbors
- Growth–share matrix — 0.80
- Niche Market — 0.78
- Demand curve — 0.78
- Divisia Index — 0.78
- Sales cannibalization — 0.78
Computed from structural-signature embeddings · 2026-10-17
References¶
[^ref-6e9fa2b4f16c]: Procter & Gamble, Form 10-Q for the quarter ended December 31, 2024, MD&A, printed pp. 15 and 24; underlying share data typically lag by one or two months. [^ref-b22ded44006e]: European Commission, Case M.9076 — Novelis/Aleris, C(2019) 7061 final, §8.3.4.3, recitals 417–430, printed pp. 71–73. [^ref-ea301e7803d7]: US Department of Justice and Federal Trade Commission, 2023 Merger Guidelines, §4.3, Market Definition. [^ref-ea301e7803d7-2]: US Department of Justice and Federal Trade Commission, 2023 Merger Guidelines, §4.4, Calculating Market Shares and Concentration, especially §4.4.B. [^ref-2a71617345a3]: US Department of Justice, Herfindahl–Hirschman Index.