PSUs are granted to executives, have a three year cycle and are granted as a target number of units subject to adjustment. The number of shares issued when PSUs vest is determined based on the achievement of performance goals and based on BMS’s three-year total shareholder return relative to a peer group of companies. Vesting is conditioned upon continuous employment and occurs on the third anniversary of the grant date. Stock-based compensation expense for awards ultimately expected to vest is recognized over the vesting period. Forfeitures are estimated based on historical experience at the time of grant and revised in subsequent periods if actual forfeitures differ from those estimates. Stock-based compensation expense was as follows: (a) Income tax benefit excludes excess tax benefits from share-based compensation awards that were vested or exercised of $35 million in 2020, $4 million in 2019 and $25 million in 2018. The total stock-based compensation expense for the years ended December 31, 2020 and 2019 includes $382 million and $66 million, respectively, related to Celgene post-combination service period and $71 million and $145 million, respectively, of accelerated vesting of awards related to the Celgene acquisition. It also includes $3 million in 2020 and $10 million in 2019 related to CVR obligation on unvested stock awards for the post combination service period. Refer to “—Note 4. Acquisitions, Divestitures, Licensing and Other Arrangements” for more information related to the Celgene acquisition. The replacement stock options granted to Celgene option holders on acquisition were issued consistent with the vesting conditions of the replaced award. Replacement stock options have contractual terms of 10 years from the initial grant date. The majority of stock options outstanding vest in one-fourth increments over a four year period, although certain awards cliff vest or have longer or shorter service periods. Celgene option holders may elect to exercise options at any time during the option term. However, any shares so purchased which have not vested as of the date of exercise shall be subject to forfeiture, which will lapse in accordance with the established vesting time period. The fair value on the acquisition date attributable to post-combination service, adjusted for estimated forfeitures, is recognized as expense on a straight-line basis over the remaining vesting period. BMS estimated the fair value of replacement options, using a Black-Scholes Option pricing model, with the following assumptions: The risk-free interest rate is based on rates available for U.S. Federal Reserve treasury constant maturities with a remaining term equal to the options' expected life at the time of the replacement award. Expected volatility of replacement stock option awards was estimated based on a 50/50 blend of implied volatility and five year historical volatility of BMS’ publicly traded stocks. The expected term of an employee share option is the period of time for which the option is expected to be outstanding and is based on historical and forecasted exercise behavior. Dividend yield is estimated based on BMS’ annual dividend rate at the time of award replacement. Year Ended December 31, Dollars in Millions 2020 2019 2018 Cost of products sold $ 37  $ 19  $ 15  Marketing, selling and administrative 332  162  122  Research and development 339  115  84  Other (income)/expense, net 71  145  —  Total stock-based compensation expense $ 779  $ 441  $ 221  Income tax benefit $ 158  $ 87  $ 41  (a) Year Ended December 31, 2019 Weighted average risk-free interest rate 1.59% Expected volatility 25.7% Weighted average expected term (years) 2.65 Expected dividend yield 2.89% 111