This excerpt taken from the NDAQ 10-K filed Feb 27, 2009.
Critical Accounting Policies and Estimates
The preparation of Consolidated Financial Statements in conformity with U.S. GAAP requires management to use judgment in making estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities and the reported amounts of revenue and expenses. The following critical accounting policies are based on, among other things, judgments and assumptions made by management that include inherent risk and uncertainties. Managements estimates are based on the relevant information available at the end of each period. For a summary of our significant accounting policies, including the accounting policies discussed below, see Note 2, Summary of Significant Accounting Policies, to the consolidated financial statements for further discussion.