The three market structures are perfect competition, monopolistic competition, and oligopoly. They seem to depend on the number of firms in the industry and level of product differentiation. Oligopoly is characterized by having a few large firms, similar but not identical products, and interdependent decision making. The concentration ratio measures the percentage of total market output produced by the top four firms, with higher ratios indicating less competition. Several US industries have very high concentration ratios, including video game consoles at 100% and credit cards at 99%.