and purchases under employee stock purchase plans, in accordance with ASC 718, Compensation—Stock Compensation, which requires that share-based payments (to the extent they are compensatory) be recognized in our consolidated statements of operations based on their fair values. We recognize stock-based compensation expense on a straight-line basis over the service period of the award, which is generally four years.

We are required to estimate the stock awards that we ultimately expect to vest and to reduce stock-based compensation expense for the effects of estimated forfeitures of awards over the expense recognition period. Although we estimate the rate of future forfeitures based upon historical experience, actual forfeitures in the future may differ. To the extent our actual forfeitures are different than our estimates, we record a true-up for the difference in the period that the awards vest and such true-ups could materially affect our operating results. Additionally, we also consider on a quarterly basis whether there have been any significant changes in facts and circumstances that would affect our expected forfeiture rate.

We estimate the fair values of employee stock options using a Black-Scholes-Merton valuation model. The fair value of an award is affected by our stock price on the date of grant as well as other assumptions including the estimated volatility of our stock price over the term of the awards and the estimated period of time that we expect employees to hold their stock options. The risk-free interest rate assumption we use is based upon U.S. treasury interest rates appropriate for the expected life of the awards. We use the implied volatility of publicly traded options in our stock in order to estimate future stock price trends as we believe that implied volatility is more representative of future stock price trends than historical volatility. In order to determine the estimated period of time that we expect employees to hold their stock options, we have used historical rates of employee groups by seniority of job classification. Our expected dividend rate is based upon an annualized dividend yield based on the per share dividend declared by our Board of Directors. The aforementioned inputs entered into the option valuation model we use to fair value our stock awards are subjective estimates and changes to these estimates will cause the fair values of our stock awards and related stock-based compensation expense that we record to vary.

We record deferred tax assets for stock-based compensation awards that result in deductions on our income tax returns, based on the amount of stock-based compensation recognized and the fair values attributable to the vested portion of stock awards assumed in connection with a business combination, at the statutory tax rate in the jurisdiction in which we will receive a tax deduction. Because the deferred tax assets we record are based upon the stock-based compensation expenses in a particular jurisdiction, the aforementioned inputs that affect the fair values of our stock awards may also indirectly affect our income tax expense. In addition, differences between the deferred tax assets