This excerpt taken from the SIAL 10-K filed Mar 14, 2005.
CRITICAL ACCOUNTING POLICIES
The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States requires management to use judgment in making estimates and assumptions that affect the reported amounts of assets and liabilities, disclosures of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the years presented. Actual results could differ from those estimates under different assumptions or conditions.
The following accounting policies are based on, among other things, judgments and assumptions made by management that include inherent risks and uncertainties. Managements estimates are based on the relevant information available at the end of each period.