(d) We define net operating cash flow as net cash provided by operating activities less purchases of property and equipment. Free cash flow is defined as net operating cash flow less dividends paid. We believe net operating cash flow and free cash flow are meaningful measures as they are commonly utilized by management and investors to assess our ability to generate cash flow from business operations to repay debt and return capital to our stockholders. Net operating cash flow is also a key metric used in our calculation of incentive compensation. Other companies may calculate net operating cash flow and free cash flow differently.
(e) We believe that ROIC is a meaningful overall measure of business performance because it reflects the company's earnings performance relative to its investment level. ROIC is also a key metric used in our calculation of incentive compensation. Other companies may calculate ROIC differently.
(f) After-tax net interest expense is equal to multiplying net interest expense by the applicable effective income tax rate for each presented quarter.
(g) Average invested capital is equal to the average of invested capital at the beginning of the year and end of the year.
(h) After-tax ROIC is equal to after-tax return divided by average invested capital.