We assess the impairment of long-lived assets when events or changes in circumstances indicate that the carrying value of the assets or the asset grouping may not be recoverable. Factors that we consider in deciding when to perform an impairment review include significant under-performance of a business or product line in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in our use of the assets. Recoverability of assets that will continue to be used in our operations is measured by comparing the carrying amount of the asset grouping to our estimate of the related total future undiscounted net cash flows. If an asset grouping's carrying value is not recoverable through the related undiscounted cash flows, the asset grouping is considered to be impaired. The impairment is measured by comparing the difference between the asset grouping's carrying amount and its fair value, based on the best information available, including market prices or discounted cash flow analysis.

Impairments of long-lived assets are determined for groups of assets related to the lowest level of identifiable independent cash flows. Due to our asset usage model and the interchangeable nature of our semiconductor manufacturing capacity, we must make subjective judgments in determining the independent cash flows that can be related to specific asset groupings. In addition, as we make manufacturing process conversions and other factory planning decisions, we must make subjective judgments regarding the remaining useful lives of assets, primarily process-specific semiconductor manufacturing tools and building improvements. When we determine that the useful lives of assets are shorter than we had originally estimated, we accelerate the rate of depreciation over the assets' new, shorter useful lives. Over the past 12 quarters, including the fourth quarter of 2007, impairments and accelerated depreciation of long-lived assets have ranged between $1 million and $320 million per quarter. This range includes restructuring charges for asset impairments between zero and $317 million per quarter.