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 $30 million impact on our 2014 operating income. Our estimated forfeiture rate as of December 31, 2014 and 2013 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 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., many countries in the European Union, and a number of other countries are actively considering 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. operations, we would be required to accrue or pay U.S. taxes on some or all of these undistributed earnings and our effective tax rate would be adversely affected. We are also subject to audit in various jurisdictions, and such jurisdictions may assess additional income tax liabilities against us. In addition, in October 2014, the European Commission opened a formal investigation to examine whether decisions by the tax authorities in Luxembourg with regard to the corporate income tax paid by certain of our subsidiaries comply with European Union rules on state aid. If this matter is adversely resolved, Luxembourg may be required to assess, and we may be required to