We generally allocate corporate overhead expenses to excluded sales centers on the basis of their net sales as a percentage of total net sales. After 15 months of operations, we include acquired, consolidated and new market sales centers in the base business calculation including the comparative prior year period.
The table below summarizes the changes in our sales centers during 2012:
| December 31, 2011
| New locations
| December 31, 2012
We define Adjusted EBITDA as net income or net loss plus interest expense, income taxes, depreciation, amortization, share‑based compensation, goodwill and other non-cash impairments and equity earnings or losses in unconsolidated investments. Adjusted EBITDA is not a measure of cash flow or liquidity as determined by generally accepted accounting principles (GAAP). We have included Adjusted EBITDA as a supplemental disclosure because we believe that it is widely used by our investors, industry analysts and others as a useful supplemental liquidity measure in conjunction with cash flows provided by or used in operating activities to help investors understand our ability to provide cash flows to fund growth, service debt and pay dividends as well as compare our cash flow generating capacity from year to year.
We believe Adjusted EBITDA should be considered in addition to, not as a substitute for, operating income or loss, net income or loss, cash flows provided by or used in operating, investing and financing activities or other income statement or cash flow statement line items reported in accordance with GAAP. Other companies may calculate Adjusted EBITDA differently than we do, which may limit its usefulness as a comparative measure.