What amounts to a substantial lessening of, or significant impediment to competition is usually answered through empirical study. The market shares of the merging companies can be assessed and added, although this kind of analysis only gives rise to presumptions, not conclusions. The Herfindahl-Hirschman Index is used to calculate the "density" of the market, or what concentration exists. Aside from the maths, it is important to consider the product in question and the rate of technical innovation in the market. A further problem of collective dominance, or oligopoly through "economic links" can arise, whereby the new market becomes more conducive to collusion. It is relevant how transparent a market is, because a more concentrated structure could mean firms can coordinate their behaviour more easily, whether firms can deploy deterrents and whether firms are safe from a reaction by their competitors and consumers. The entry of new firms to the market, and any barriers that they might encounter should be considered.