Year ended December 31 | |||||||||||||
2013 | 2012 | 2011 | |||||||||||
Stock Options | |||||||||||||
Expected term in years1 | 6.0 | 6.0 | 6.2 | ||||||||||
Volatility2 | 31.3 | % | 31.7 | % | 31.0 | % | |||||||
Risk-free interest rate based on zero coupon U.S. treasury note | 1.2 | % | 1.1 | % | 2.6 | % | |||||||
Dividend yield | 3.3 | % | 3.2 | % | 3.6 | % | |||||||
Weighted-average fair value per option granted | $ | 24.48 | $ | 23.35 | $ | 21.24 |
1 | Expected term is based on historical exercise and postvesting cancellation data. |
2 | Volatility rate is based on historical stock prices over an appropriate period, generally equal to the expected term. |