This excerpt taken from the C 10-K filed Feb 22, 2008.
Adoption of SFAS 157Fair Value Measurements
The Company elected to early-adopt SFAS No. 157, Fair Value Measurements (SFAS 157), as of January 1, 2007. SFAS 157 defines fair value, expands disclosure requirements around fair value and 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 Companys market assumptions. These two types of inputs create the following fair value hierarchy: