SAS 57 Section 342 Auditing Accounting Estimates

This section provides guidance to auditors on obtaining and evaluating
sufficient appropriate audit evidence to support significant accounting estimates
in an audit of financial statements in accordance with generally accepted
auditing standards. For purposes of this section, an accounting estimate is an
approximation of a financial statement element, item, or account. Accounting
estimates are often included in historical financial statements because

a. The measurement of some amounts or the valuation of some accounts
is uncertain, pending the outcome of future events.

b. Relevant data concerning events that have already occurred cannot be
accumulated on a timely, cost-effective basis.