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The audit market for large public companies is an oligopoly, with the largest firms auditing the vast majority of public companies and smaller firms facing significant barriers to entry into the market. Mergers among the largest firms in the 1980s and 1990s and the dissolution of Arthur Andersen in 2002 significantly increased concentration among the largest firms, known as the "Big 4." These four firms currently audit over 78 percent of all U.S. public companies and 99 percent of all public company sales. This consolidation and the resulting concentration have raised a number of concerns. To address them, the Sarbanes-Oxley Act of 2002 mandated that GAO study (1) the factors contributing to the mergers; (2) the implications of consolidation on competition and client choice, audit fees, audit quality, and auditor independence; (3) the impact of consolidation on capital formation and securities markets; and (4) barriers to entry faced by smaller accounting firms in competing with the largest firms for large public company audits.

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