📖 Glossary

Market Share — Definition, the Double Jeopardy Law & How Brands Grow

Market share is a brand's percentage of total category sales. Here's what the Ehrenberg-Bass evidence says about how it actually moves — penetration, double jeopardy, share of voice — and the common errors.

·5 min read

Definition (≤25 words). Market share is a brand's percentage of total category sales, measured by value or volume — the standard scoreboard of competitive position and growth.

TL;DR. Market share is the number every marketing plan is trying to move and most misunderstand. The Ehrenberg-Bass Institute's evidence, gathered across decades and categories, shows that share is driven overwhelmingly by market penetration — how many people buy you — not by how loyal each buyer is, and that share follows predictable laws (double jeopardy, the Dirichlet model) that most growth strategies ignore. It moves slowly, through reach and mental availability, and almost never through loyalty-deepening alone. Understanding what actually shifts it is the difference between a growth strategy and a wish.

What is market share, precisely?

Market share is your brand's sales expressed as a percentage of total category sales, over a defined period. It can be measured by value (revenue) or volume (units), and the two can diverge — a brand can hold volume share while losing value share if it is discounting heavily, which is why profit-focused teams watch both. The category definition matters too: share in "cola" is a different contest from share in "soft drinks," and drawing the boundary too narrowly is a common way to flatter yourself.

Share is the headline number of competitive position because it is comparatively hard to fake — it sits on top of real sales in a real market, not on a tracking-study score. It is also stubbornly stable in most categories, which is itself the most important thing to know about it.

What actually drives market share?

This is where the evidence overturns most of the marketing folklore. The Ehrenberg-Bass Institute's work — Byron Sharp's How Brands Grow (2010) and Jenni Romaniuk and Sharp's How Brands Grow 2 (2016) — built on decades of panel-data research to reach a clear, repeatedly replicated conclusion:

Market share is driven by market penetration — the number (and weight) of buyers a brand has — not by the depth of their loyalty.

The implication is large and uncomfortable for a lot of strategy. It means:

  1. Big brands have more buyers, not more loyal buyers. A market leader and a small brand have customers who behave in surprisingly similar ways — buying at similar rates, defecting at similar rates. What separates them is how many customers each has.

  2. Growth means adding buyers, mostly light ones. To grow share, a brand must reach more of the category — particularly the large majority of light and occasional buyers — rather than squeezing more from its devoted core. This is why broad-reach brand building, not CRM intensification, is the engine of marketing effectiveness.

  3. Loyalty metrics are governed by the double jeopardy law. Smaller brands suffer twice: they have fewer buyers and those buyers buy slightly less often. You cannot escape double jeopardy by trying to make your small base more loyal; the law says loyalty follows share, not the other way around.

This is why the Ehrenberg-Bass prescription for share growth is reach, mental availability, and physical availability — the things that put the brand in front of more buyers in more buying situations — rather than the loyalty and relationship programmes that dominate so many plans.

How does market share move over time?

Two empirical regularities matter:

Stationarity

In established categories, market shares are remarkably stable. Year on year, the ranking barely shifts, and big swings are rare. This is not because brands are not trying; it is because the underlying buyer behaviour — the repertoire buying, the defection rates, the penetration patterns — is itself stable. The Dirichlet model formalises that stability: it predicts the penetration and loyalty metrics a brand of any given share should exhibit, and real markets conform to it closely — which is why share forecasts that ignore the model tend to be optimistic.

Share of voice and excess share of voice

The link between marketing investment and share runs through share of voice. The long-standing finding, reinforced by Binet and Field's IPA Databank work, is that a brand's market share tends to track its share of voice over the long term. Excess share of voice — spending above your current market share — is associated with share growth; spending below it is associated with share decline. This is the mechanism behind most sustained-growth strategies: invest above your weight class, consistently, and the penetration economics do the rest. It is also why cutting marketing in a downturn, to protect short-term profit, so often costs share that takes years to recover.

Where people get it wrong

  • Chasing loyalty to grow share. The evidence says loyalty is a consequence of share, not a cause. Programmes that try to grow share by deepening the loyalty of an existing base fight against the double jeopardy law and usually fail to move the headline number.
  • Defining the category to suit the story. "Number one in premium ambient breakfast cereal" is a phrase designed to hide a small share in a bigger market. Define the category the way buyers shop it, or the number is meaningless.
  • Confusing volume share with value share. You can hold volume share while destroying value share through discounting. If you are buying volume with margin, your "stable share" may be masking a deteriorating business.
  • Expecting fast share movement. Stationarity is the norm. A strategy that promises a step-change in share inside a year is almost always over-promising — or buying it with unsustainable promotion.
  • Ignoring excess share of voice. Share growth requires spending above your share. Plans that aim to grow share while cutting share of voice are working against the one mechanism that reliably moves it.

In practice — how to actually grow share

  1. Grow penetration, not loyalty. Aim your investment at reaching more category buyers — especially the light, occasional majority — because that is what the evidence says moves share.
  2. Build mental availability. Be thought of in more buying situations, across more category entry points, so you are in the consideration set of buyers who are not yet yours.
  3. Maintain excess share of voice. If you want to grow share, spend above it — consistently, over years. Track share of voice against share of market as a leading indicator.
  4. Watch value and volume separately. A share number that is only propped up by discounting is not growth; it is margin erosion disguised as momentum.
  5. Define the category honestly. Use the category boundary your buyers actually use, and benchmark against the real competitive set, not a flattering subset.
  6. Be patient. Share moves slowly. Sustained, broad-reach brand building is the proven path; the shortcuts are usually promotions that borrow share from the future and hand it back with interest.

So what?

Market share is the scoreboard, but the way most teams try to move it — loyalty, CRM, narrow targeting — is the opposite of what the evidence supports. Brands grow share the boring way: by reaching more buyers, building the mental availability that puts them in the choice set, and spending above their weight consistently enough for excess share of voice to do its work. There is no hack, no loyalty revolution, no clever redefinition of the category that substitutes for that. The brands that grow are the ones that accept the laws — double jeopardy, stationarity, the penetration-driven engine — and invest patiently against them. That is the unglamorous truth, and it is the whole point of this site.

Evidence & sources

  • Sharp, B. (2010). How Brands Grow. Oxford University Press — penetration as the driver of share; double jeopardy; the Dirichlet model; stationarity.
  • Romaniuk, J. & Sharp, B. (2016). How Brands Grow 2. Oxford University Press — mental availability and category entry points as the mechanism behind penetration growth.
  • Ehrenberg-Bass Institute — duplication of purchase law; long-running panel-data evidence on share stability.
  • Binet, L. & Field, P. (2013). The Long and the Short of It. IPA — share of voice and excess share of voice as predictors of share movement.
  • Goodhardt, G.J., Ehrenberg, A.S.C. & Chatfield, C. (1984). "The Dirichlet: A Comprehensive Model of Buying Behaviour." Journal of the Royal Statistical Society, Series A, 147(5), 621–655 — the formal model behind double jeopardy and share stationarity.

Definition type: DefinedTerm. Reviewed: 2026-07-09. Written by Andrei Tarnovski for Boringly Effective. Fundamentals over fashion.