on the date of grant using a Black-Scholes model. The estimated number of stock awards that will ultimately vest requires judgment, 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 level, economic conditions, time remaining to vest, and historical forfeiture 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 $46 million impact on our 2015 operating income. Our estimated forfeiture rates as of December 31, 2015 and 2014 , were 28% and 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 first year. If forfeited early in the life of an award, the compensation expense adjustment is much greater under an accelerated method than under a straight-line method. 

Income Taxes 

We are subject to income taxes in the U.S. (federal and state) and numerous foreign jurisdictions. Tax laws, regulations, and administrative practices in various jurisdictions may be subject to significant change due to economic, political, and other conditions, and significant judgment is required in evaluating and estimating our provision and accruals for these taxes. There are many transactions that occur during the ordinary course of business for which the ultimate tax determination is uncertain. Our effective tax rates could be adversely affected by earnings being lower than anticipated in jurisdictions where we have lower statutory rates and higher than anticipated in jurisdictions where we have higher statutory rates, losses incurred in jurisdictions for which we are not able to realize the related tax benefit, changes in foreign currency exchange rates, entry into new businesses and geographies and changes to our existing businesses, acquisitions (including integrations) and investments, changes in our deferred tax assets and liabilities and their valuation, and changes in the relevant tax, accounting, and other laws, regulations, administrative practices, principles, and interpretations, including fundamental changes to the tax laws applicable to corporate multinationals. The U.S., the European Union and its member states, and a number of other countries are actively pursuing changes in this regard. 

Except as required under U.S. tax laws, we do not provide for U.S. taxes on our undistributed earnings of foreign subsidiaries that have not been previously taxed since we intend to invest such undistributed earnings indefinitely outside of the U.S. If our intent changes or if these funds are needed for our U.S.