We must make certain estimates and judgments in determining income tax expense for financial statement purposes. These estimates and judgments occur in the calculation of tax credits, benefits, and deductions, and in the calculation of certain tax assets and liabilities, which arise from differences in the timing of recognition of revenue and expense for tax and financial statement purposes, as well as the interest and penalties related to these uncertain tax positions. Significant changes to these estimates may result in an increase or decrease to our tax provision in a subsequent period.

We must assess the likelihood that we will be able to recover our deferred tax assets. If recovery is not likely, we must increase our provision for taxes by recording a valuation allowance against the deferred tax assets that we estimate will not ultimately be recoverable. We believe that we will ultimately recover a substantial majority of the deferred tax assets recorded on our consolidated balance sheets. However, should there be a change in our ability to recover our deferred tax assets, our tax provision would increase in the period in which we determined that the recovery was not likely.

The calculation of our tax liabilities involves dealing with uncertainties in the application of complex tax regulations. In the first quarter of 2007, we adopted Financial Accounting Standards Board (FASB) Interpretation No. 48, "Accounting for Uncertainty in Income Taxes—an interpretation of SFAS No. 109" (FIN 48), and related guidance (see "Note 17: Taxes" in Part II, Item 8 of this Form 10-K). As a result of the implementation of FIN 48, we recognize liabilities for uncertain tax positions based on the two-step process prescribed in the interpretation. The first step is to evaluate the tax position for recognition by determining if the weight of available evidence indicates that it is more likely than not that the position will be sustained on audit, including resolution of related appeals or litigation processes, if any. The second step requires us to estimate and measure the tax benefit as the largest amount that is more than 50% likely to be realized upon ultimate settlement. It is inherently difficult and subjective to estimate such amounts, as we have to determine the probability of various possible outcomes. We reevaluate these uncertain tax positions on a quarterly basis. This evaluation is based on factors including, but not limited to, changes in facts or circumstances, changes in tax law, effectively settled issues under audit, and new audit activity. Such a change in recognition or measurement would result in the recognition of a tax benefit or an additional charge to the tax provision.