on our best estimate of future net sales and operating expenses, based primarily on expected category expansion, pricing, market segment share, and general economic conditions. Certain estimates of discounted cash flows involve businesses and geographies with limited financial history and developing revenue models. Changes in these forecasts could significantly change the amount of impairment recorded, if any. 

During the year, management monitored the actual performance of the business relative to the fair value assumptions used during our annual goodwill impairment test. For the periods presented, no triggering events were identified that required an update to our annual impairment test. As a measure of sensitivity, a 10% decrease in the fair value of any of our reporting units as of December 31, 2013 would have had no impact on the carrying value of our goodwill. 

Financial and credit market volatility directly impacts our fair value measurement through our weighted average cost of capital that we use to determine our discount rate and through our stock price that we use to determine our market capitalization. During times of volatility, significant judgment must be applied to determine whether credit or stock price changes are a short-term swing or a longer-term trend. We have not made any significant changes to the accounting methodology used to evaluate goodwill impairment. Changes in our estimated future cash flows and asset fair values may cause us to realize material impairment charges in the future. As a measure of sensitivity, a prolonged 20% decrease from our December 31, 2013 closing stock price would not be an indicator of possible impairment. 

Stock-Based Compensation 

We measure compensation cost for stock awards at fair value and recognize it as compensation expense over the service period for awards expected to vest. The fair value of restricted stock units is determined based on the number of shares granted and the quoted price of our common stock. The estimation of stock awards that will ultimately vest requires judgment for the amount that will be forfeited, and to the extent actual results or updated estimates differ from our current estimates, such amounts will be recorded as a cumulative adjustment in the period estimates are revised. We consider many factors when estimating expected forfeitures, including employee class, economic environment, and historical experience. We update our estimated forfeiture rate quarterly. We have not made any significant changes to the accounting methodology used to evaluate stock-based compensation. Changes in our estimates and assumptions may cause us to realize material changes in stock-based compensation expense in the future. As a measure of sensitivity, a 1% change to our estimated forfeiture rate would have had an approximately $32 million impact on our 2013 operating income. Our estimated forfeiture rate as of December 31, 2013 and 2012 was 27%. 

We utilize the accelerated method, rather than the straight-line method, for recognizing compensation expense. For example, over 50% of the compensation cost related to an award vesting ratably over four years is expensed in the