remain minor relative to our initial investment. This is a trend we expect to continue. In addition, depreciation is also based on the estimated useful lives of our IBX data centers. These estimates could vary from actual performance of the asset, are based on historical costs incurred to build out our IBX data centers and are not indicative of current or expected future capital expenditures. Therefore, we exclude depreciation from our operating results when evaluating our operations.
In addition, in presenting adjusted EBITDA and AFFO, we exclude amortization expense related to acquired intangible assets. Amortization expense is significantly affected by the timing and magnitude of our acquisitions and these charges may vary in amount from period to period. We exclude amortization expense to facilitate a more meaningful evaluation of our current operating performance and comparisons to our prior periods. We exclude accretion expense, both as it relates to asset retirement obligations as well as accrued restructuring charge liabilities, as these expenses represent costs which we believe are not meaningful in evaluating our current operations. We exclude stock-based compensation expense, as it can vary significantly from period to period based on share price, the timing, size and nature of equity awards. As such, we, and many investors and analysts, exclude stock-based compensation expense to compare our operating results with those of other companies. We also exclude restructuring charges. The restructuring charges relate to our decisions to exit leases for excess space adjacent to several of our IBX data centers, which we did not intend to build out, or our decision to reverse such restructuring charges. We also exclude impairment charges related to certain long-lived assets. The impairment charges are related to expense recognized whenever events or changes in circumstances indicate that the carrying amount of long-lived assets are not recoverable. We also exclude gain or loss on asset sales as it represents profit or loss that is not meaningful in evaluating the current or future operating performance. Finally, we exclude acquisition costs from AFFO and adjusted EBITDA to allow more comparable comparisons of our financial results to our historical operations. The acquisition costs relate to costs we incur in connection with business combinations. Such charges generally are not relevant to assessing the long-term performance of the company. In addition, the frequency and amount of such charges vary significantly based on the size and timing of the acquisitions. Management believes items such as restructuring charges, impairment charges, gain or loss on asset sales and acquisition costs are non-core transactions; however, these types of costs may occur in future periods.
We define adjusted EBITDA as income or loss from operations excluding depreciation, amortization, accretion, stock-based compensation expense, restructuring charges, impairment charges, acquisition costs, and gain on asset sales as presented below (in thousands):
Three Months Ended
Income from operations
Depreciation, amortization, and accretion expense
Stock-based compensation expense
Our adjusted EBITDA results have improved each year and in each region in total dollars due to the improved operating results discussed earlier in "Results of Operations", as well as the nature of our business model consisting of a recurring revenue stream and a cost structure which has a large base that is fixed in nature also discussed earlier in "Overview".
Funds from Operations ("FFO") and AFFO
We use FFO and AFFO, which are non-GAAP financial measures commonly used in the REIT industry. FFO is calculated in accordance with the standards established by the National Association of Real Estate Investment Trusts. FFO represents net income (loss), excluding gain (loss) from the disposition of real estate assets, depreciation and amortization on real estate assets and adjustments for unconsolidated joint ventures' and non-controlling interests' share of these items.
In presenting AFFO, we exclude certain items that we believe are not good indicators of our current or future operating performance. AFFO represents FFO excluding depreciation and amortization expense on non-real estate assets, accretion, stock-based compensation, restructuring charges, impairment charges, acquisition costs, an installation revenue adjustment, a straight-line rent expense adjustment, a contract cost adjustment, amortization of deferred financing costs and debt discounts and premiums, gain (loss) on debt extinguishment, an income tax expense adjustment, recurring capital expenditures and adjustments for unconsolidated joint ventures' and noncontrolling interests' share of these items and net income (loss) from discontinued operations, net of tax. The adjustments for installation revenue, straight-line rent expense and contract costs are intended to isolate the cash activity included within the straight-lined or amortized results in the condensed consolidated statement of operations. We exclude