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performance-based RSUs. The fair value of restricted stock issued is equal to the closing price of our stock on the date of grant. We will continue to assess the assumptions and methodologies used to calculate the estimated fair value of stock-based compensation. If circumstances change, and additional data becomes available over time, we may change our assumptions and methodologies, which may materially impact our fair value determination. 

The following table summarizes stock-based compensation expense related to stock options, RSUs and restricted stock for the years ended December 31, 2013, 2012 and 2011, which was recognized as follows: 

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At December 31, 2013, total compensation cost related to non-vested stock options not yet recognized was approximately $17.1 million, which is expected to be recognized over an average remaining recognition period of 2.8 years. 

The stock option pricing model requires the use of several assumptions that impact the fair value estimate. These variables include, but are not limited to, the volatility of our stock price and employee exercise behaviors. There were no material changes made during 2013 to the methodology used to determine our assumptions. 

The weighted-average estimated fair value of stock options granted to employees and directors during the years ended December 31, 2013, 2012 and 2011 was $8.35 per share, $5.60 per share and $9.53 per share, respectively, with the following weighted-average assumptions: 

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We based our estimate of expected volatility for the years ended December 31, 2013, 2012 and 2011 on the sequential historical daily trading data of our common stock for a period equal to the expected life of the options granted. The selection of the historical volatility method was based on available data indicating our historical volatility is as equally representative of our future stock price trends as is our implied volatility. We have no reason to believe the future volatility of our stock price is likely to differ from its past volatility. 

The risk-free interest rate assumption is based upon implied yields of U.S. Treasury zero-coupon bonds on the date of grant having a remaining term equal to the expected life of the options granted. The dividend yield is based on our historical and expected dividend payouts. 

The expected life of our stock options is based upon historical exercise and cancellation activity of our previous stock-based grants with a ten-year contractual term. 

 The RSU pricing model also requires the use of several significant assumptions that impact the fair value estimate. The estimated fair value of the RSUs granted to employees during the years ended December 31, 2013, 2012 and 2011 was $27.72 per share, $19.46 per share and $38.73 per share, respectively, with the following assumptions: 

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