One common measure of market concentration is the fraction of total industry sales produced by the top four firms in the industry. The closer this fraction is to 1, the higher the concentration ratio.

Among the roles of managers is to make price and output decisions. This decision depends on the size and distribution of firms within the industry. Managers who face little competition from others in the industry can have the ability to control production and raise the price. The Healthcare industry in the US is a good example of high market concentration. As you have indicated, Following Obamacare, mergers of the hospital led to market power and raised healthcare costs.

Why is healthcare cost in the US is much higher than advanced economies and yet there are many who no healthcare coverage?