trade volume of our tradable forward call options prior to 2011 precluded sole reliance on implied volatility, and as such we include historical volatility in our computation of expected volatility. An increase/decrease of 10% in our computation of expected volatility would increase/decrease the total stock-based compensation expense by approximately $10.9 million for the year ended December 31, 2014.

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Suboptimal Exercise Factor: Our computation of the suboptimal exercise factor is based on historical option exercise behavior and is determined for both executives and non-executives. An increase/decrease in the suboptimal exercise factor of 10% would increase/decrease the total stock-based compensation expense by approximately $2.4 million for the year ended December 31, 2014.