2011 Activity Stock Options Stock options vest over three years and have 10-year terms. Exercise prices of stock options awarded for all periods were equal to the market price of the stock on the date of grant. The following table pertains to stock options that were granted, vested, and exercised in 2011, 2010, and 2009: We estimate the fair value for stock options at the date of grant using the Black-Scholes option pricing model, which requires us to make certain assumptions. We base the risk-free interest rate on U.S. Treasury zero-coupon issues with a remaining term equal to the expected life assumed at the date of grant. The dividend yield is determined based on estimated dividend payments and changes to our stock price during the expected option life. We estimate volatility based on the historical volatility of our daily stock price over the past five years, which is commensurate with the expected life of the options. We base the average expected life on the contractual term of the stock option, historical trends in employee exercise activity, and post-vesting employment termination trends. We estimate forfeitures at the date of grant based on historical experience. The impact of forfeitures is not material. We used the following weighted average assumptions in the Black-Scholes option pricing model to determine the fair values of stock-based compensation awards during 2011, 2010, and 2009: The following table summarizes activity related to nonvested RSUs during 2011: RSUs The following table summarizes stock option activity during 2011: 76 Number of RSUs (In thousands) Weighted Average Grant-Date Fair Value Per Share Nonvested at December 31, 2010 3,756 $82.53 Granted 2,021 79.21 Vested (1,122) 94.41 Terminated (353) 77.81 Nonvested at December 31, 2011 4,302 $78.25 2011 2010 2009 Risk-free interest rate 1.97% 2.49% 1.69% Dividend yield 4.20% 3.40% 2.30% Volatility factors 0.277 0.272 0.244 Expected option life 5 years 5 years 5 years (In millions, except for grant-date fair value of stock options) 2011 2010 2009 Weighted average grant-date fair value of stock options granted $13.06 $14.05 $14.91 Aggregate fair value of all the stock options that vested 60 71 72 Aggregate intrinsic value of all of the stock options exercised 60 50 37 Number of Stock Options (In thousands) Weighted Average Exercise Price Weighted Average Remaining Contractual Life (In years) Aggregate Intrinsic Value (In millions) Outstanding at December 31, 2010 24,497 $75.90 Granted 2,540 79.60 Exercised (2,257) 51.56 Terminated (221) 83.77 Outstanding at December 31, 2011 24,559 78.45 5.7 $204.1 Vested and expected-to-vest at December 31, 2011 24,476 78.45 5.6 204.0 Vested at December 31, 2011 18,356 78.41 4.8 187.1