accordance with FASB Statement No. 123 (revised 2004), Share-Based Payment, 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. In addition, we have applied the provisions of the SEC’s Staff Accounting Bulletin No. 107 in our accounting for Statement 123(R).

 

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 forfeitures based on historical experience, actual forfeitures in the future may differ. In addition, 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.

 

As required by Statement 123(R), we recognize stock-based compensation expense for share-based payments issued or assumed after June 1, 2006 that are expected to vest. For all share-based payments granted or assumed beginning June 1, 2006, we recognize stock-based compensation expense on a straight-line basis over the service period of the award, which is generally four years. The fair value of the unvested portion of share-based payments granted prior to June 1, 2006 is recognized over the remaining service period using the accelerated expense attribution method, net of estimated forfeitures. In determining whether an award is expected to vest, we use an estimated, forward-looking forfeiture rate based upon our historical forfeiture rates. Stock-based compensation expense recorded using an estimated forfeiture rate is updated for actual forfeitures quarterly. We also consider, each quarter, whether there have been any significant changes in facts and circumstances that would affect our forfeiture rate.

 

We estimate the fair value of employee stock options using a Black-Scholes 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 United States treasury interest rates appropriate for the expected life of the awards. We use the implied volatility of our publicly traded, longest-term options 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 job classification. Our expected dividend rate is zero since we do not currently pay cash dividends on our common stock and do not anticipate doing so in the foreseeable future. The aforementioned inputs entered into the option valuation model we use to fair value our stock awards are subjective estimates and changes to these