Editing Monopoly (section)

==== Market shares ====
As with collusive conduct, market shares are determined with reference to the particular market in which the company and product in question is sold. It does not in itself determine whether an undertaking is dominant but work as an indicator of the states of the existing competition within the market. The [[Herfindahl–Hirschman index|Herfindahl–Hirschman Index]] (HHI) is sometimes used to assess how competitive an industry is. It sums up the squares of the individual market shares of all of the competitors within the market. The lower the total, the less concentrated the market and the higher the total, the more concentrated the market.<ref name=":1">Whish R and others, ''Competition Law'' (8th Edition, OUP 2015)</ref> In the US, the [[merger guidelines]] state that a post-merger HHI below 1000 is viewed as not concentrated while HHIs above that will provoke further review.

By European Union law, very large market shares raise a presumption that a company is dominant, which may be rebuttable. A market share of 100% may be very rare but it is still possible to be found and in fact it has been identified in some cases, for instance the ''AAMS v Commission'' case.<ref>AAMS v Commission [2001] ECR II-3413</ref> Undertakings possessing market share that is lower than 100% but over 90% had also been found dominant, for example, Microsoft v Commission case.<ref>''Microsoft Corporation v Commission'' [2004]</ref> In the ''AKZO v Commission'' case,<ref>Case C- 62/86 ''AKZO Chemie BV v Commission'' [1991] ECR I −3359</ref> the undertaking is presumed to be dominant if it has a market share of 50%. There are also findings of dominance that are below a market share of 50%, for instance, ''United Brands v Commission'',<ref name=":0" /> it only possessed a market share of 40% to 45% and still to be found dominant with other factors. The lowest yet market share of a company considered "dominant" in the EU was 39.7%. If a company has a dominant position, then there is a special responsibility not to allow its conduct to impair competition on the common market; however, these will all falls away if it is not dominant.<ref>Case T-203/01 ''Michelin v Commission'' [2003]</ref>

When considering whether an undertaking is dominant, it involves a combination of factors. Each of them cannot be taken separately as if they are, they will not be as determinative as they are when they are combined.<ref name=":2">Guidance on Article 102 Enforcement Priorities [2009]</ref> Also, in cases where an undertaking has previously been found dominant, it is still necessary to redefine the market and make a whole new analysis of the conditions of competition based on the available evidence at the appropriate time.<ref>''Coca-Cola Co v Commission'' [2000] ECR II- 1733</ref>