our past operating results, and our forecast of future earnings, future taxable income and prudent and feasible tax planning strategies. The assumptions utilized in determining future taxable income require significant judgment and are consistent with the plans and estimates we are using to manage the underlying businesses. Actual operating results in future years could differ from our current assumptions, judgments and estimates. However, we believe that it is more likely than not that substantially all deferred tax assets recorded on our Consolidated Balance Sheets will ultimately be realized. In the event we were to determine that we would not be able to realize all or part of our net deferred tax assets in the future, an adjustment to the deferred tax assets would be charged to earnings in the period in which we make such determination.

We did not recognize certain tax benefits from uncertain tax positions within the provision for income taxes. We may recognize a tax benefit only if it is more likely than not the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized upon settlement. At December 31, 2014, our estimated gross unrecognized tax benefits were $34.8 million of which $29.2 million, if recognized, would favorably impact our future earnings. Due to uncertainties in any tax audit outcome, our estimates of the ultimate settlement of our unrecognized tax positions may change and the actual tax benefits may differ significantly from the estimates. See Note 10 of Item 8, Financial Statements and Supplementary Data for further information regarding income taxes.

Stock-Based Compensation

Stock-based compensation expense at the grant date is based on the total number of options granted and an estimate of the fair value of the awards expected to vest and is recognized as expense ratably over the requisite service period, which is the vesting period.

We calculate the fair value of our stock option grants using a lattice-binomial model. This model requires the input of highly subjective assumptions. Changes in the subjective input assumptions can materially affect the estimate of fair value of options granted and our results of operations could be impacted.

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Expected Volatility: Our computation of expected volatility is based on a blend of historical volatility of our common stock and implied volatility of tradable forward call options to purchase shares of our common stock. Our decision to incorporate implied volatility was based on our assessment that implied volatility of publicly traded options in our common stock is more reflective of market conditions and, therefore, can reasonably be expected to be a better indicator of expected volatility than historical volatility of our common stock. Low trade volume of our tradable forward call options prior to 2011 precluded sole reliance on implied volatility, and as such we include historical volatility in our computation of expected volatility.