Other Operating Expense (Income) 

Other operating expense (income), net was $171 million , $133 million , and $114 million during 2015 , 2014 , and 2013 , and was primarily related to the amortization of intangible assets. 

Income from Operations 

For the reasons discussed above, income from operations increased to $2.2 billion in 2015 , from $178 million in 2014 and $745 million in 2013 . 

We believe that income from operations is a more meaningful measure than gross profit and gross margin due to the diversity of our product categories and services. 

Interest Income and Expense 

Our interest income was $50 million , $39 million , and $38 million during 2015 , 2014 , and 2013 . We generally invest our excess cash in investment grade short- to intermediate-term fixed income securities and AAA-rated money market funds. Our interest income corresponds with the average balance of invested funds based on the prevailing rates, which vary depending on the geographies and currencies in which they are invested. 

Interest expense was $459 million , $210 million , and $141 million in 2015 , 2014 , and 2013 . The increase is primarily due to increases in our long-term debt, and capital and finance lease arrangements. 

Our long-term debt was $8.2 billion and $8.3 billion as of December 31, 2015 and 2014 . Our other long-term liabilities were $9.9 billion and $7.4 billion as of December 31, 2015 and 2014 . See Item 8 of Part II, “Financial Statements and Supplementary Data—Note 5—Long-Term Debt and Note 6—Other Long-Term Liabilities” for additional information. 

Other Income (Expense)

Other income (expense), net was $(256) million , $(118) million , and $(136) million during 2015 , 2014 , and 2013 . The primary component of other income (expense), net is related to foreign-currency gains (losses). 

Income Taxes 

Our effective tax rate is subject to significant variation due to several factors, including variability in our pre-tax and taxable income and loss and the mix of jurisdictions to which they relate, changes in how we do business, acquisitions (including integrations) and investments, audit-related developments, foreign currency gains (losses), changes in law, regulations, and administrative practices, and relative changes of expenses or losses for which tax benefits are not recognized. Additionally, our effective tax rate can be more or less volatile based on the amount of pre-tax income or loss. For example, the impact of discrete items and non-deductible expenses on our effective tax rate is greater when our pre-tax income is lower. 

We recorded a provision for income taxes of $950 million , $167 million , and $161 million in 2015 , 2014 , and 2013 . Our provision for income taxes in 2015 was higher than in 2014 primarily due to an increase in U.S. pre-tax income and increased losses in certain foreign subsidiaries for which we may not realize a tax benefit. Losses for which we may not realize a related tax benefit, primarily due to losses of foreign subsidiaries, reduce our pre-tax income without a corresponding reduction in our tax expense, and therefore increase our effective tax rate. We have recorded valuation allowances