granted and options assumed were amortized using the accelerated expense attribution method. In addition, we reduced pro forma stock compensation expense for actual forfeitures in the periods they occurred. In March 2005, the Securities and Exchange Commission issued Staff Accounting Bulletin (SAB) No. 107, which provides supplemental implementation guidance for Statement 123R. We have applied the provisions of SAB 107 in our adoption of Statement 123R. See Note 7 of Notes to Consolidated Financial Statements for information on the impact of our adoption of Statement 123R and the assumptions we use to calculate the fair value of share-based employee compensation.

 

Upon our adoption of Statement 123R, we were required to estimate the 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 estimated forfeitures based on historical experience, forfeitures in the future may differ. Under Statement 123R, the forfeiture rate must be revised if actual forfeitures differ from our original estimates. Also in connection with our adoption of Statement 123R, we elected to recognize awards granted after our adoption date under the straight-line amortization method.

 

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 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 stock 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.

 

We record deferred tax assets for stock-based awards that result in deductions on our income tax returns, based on the amount of stock-based compensation recognized and the statutory tax rate in the jurisdiction in which we will receive a tax deduction. Differences between the deferred tax assets recognized for financial reporting purposes and the actual tax deduction reported on our income tax returns are recorded in additional paid-in capital. If the tax deduction is less than the deferred tax asset, such shortfalls reduce our pool of excess tax benefits. If the pool of excess tax benefits is reduced to zero, then subsequent shortfalls would increase our income tax expense. Our pool of excess tax benefits is computed in accordance with the alternative transition method as prescribed under FASB