This excerpt taken from the C 10-Q filed Nov 5, 2007.
Fair Value Hierarchy
Statement 157 specifies a hierarchy of valuation techniques based on whether the inputs to those valuation techniques are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect the Company's market assumptions. In accordance with SFAS 157, these two types of inputs have created the following fair value hierarchy:
This hierarchy requires the use of observable market data when available.