Agilent accounts for share-based awards in accordance with the provisions of the revised accounting guidance which requires the measurement and recognition of compensation expense for all share-based payment awards made to our employees and directors including employee stock option awards, restricted stock units, employee stock purchases made under our employee stock purchase plan (ESPP) and performance share awards granted to selected members of our senior management under the long-term performance plan (LTPP) based on estimated fair values.

The impact on our results for share-based compensation was as follows:

 

 

During the three months ended April 30, 2010, we reversed approximately $3 million of expense for the cancellation of non-vested awards related to the separation of a senior executive. At April 30, 2011 there was no share-based compensation capitalized within inventory. The windfall tax benefit realized from exercised stock options and similar awards was not material for the three and six months ended April 30, 2011 and 2010.

The following assumptions were used to estimate the fair value of the options and LTPP grants. We had no employee stock options granted during the three months ended April 30, 2011.

 

The fair value of share-based awards for employee stock option awards was estimated using the Black-Scholes option pricing model. Shares granted under the LTPP were valued using a Monte Carlo simulation model. Both the Black-Scholes and Monte Carlo simulation fair value models require the use of highly subjective and complex assumptions, including the option's expected life and the price volatility of the underlying stock. The estimated fair value of restricted stock unit awards is determined based on the market price of Agilent's common stock on the date of grant. The ESPP allows eligible employees to purchase shares of our common stock at 85 percent of the purchase price and uses the purchase date to establish the fair market value.

We use historical volatility to estimate the expected stock price volatility assumption for employee stock option awards. In reaching the conclusion, we have considered many factors including the extent to which our options are currently traded and our ability to find traded options in the current market with similar terms and prices to the options we are valuing. For the grants awarded under the 2009 stock plan after November 1, 2010, we increased the period available to retirement eligible employees to exercise their options from three years at retirement date to the full contractual term of ten years. In developing our estimated life of our employee stock options of 5.8 years, we considered the historical option exercise behavior of our executive employees who were granted the majority of the options in the annual grants made during the three months ended January 31, 2011, which we believe is representative of future behavior.