increased 70 %, 49 %, and 69 % in 2015 , 2014 , and 2013 , compared to the comparable prior year periods. The sales growth primarily reflects increased customer usage, partially offset by pricing changes. Pricing changes were driven largely by our continued efforts to reduce prices for our customers. 

Segment Operating Income (Loss) 

The increase in North America segment operating income in absolute dollars in 2015 , 2014 , and 2013 , compared to the comparable prior year periods, is primarily due to increased unit sales, including sales by marketplace sellers, partially offset by increased levels of operating expenses to expand our fulfillment capacity and spending on technology infrastructure. There was a favorable impact from foreign exchange rates of $30 million, $0, and $7 million for 2015 , 2014 , and 2013 . 

The decrease in International segment operating loss in absolute dollars in 2015 , compared to the comparable prior year period, is primarily due to increased unit sales, including sales by marketplace sellers, partially offset by increased levels of operating expenses to expand our fulfillment capacity, spending on technology infrastructure and marketing efforts, and the unfavorable impact from foreign exchange rates. The unfavorable impact from foreign exchange rates was $278 million, $27 million, and $70 million for 2015 , 2014 , and 2013 . The decrease in International segment operating income (loss) in absolute dollars in 2014, compared to the comparable prior year period, is primarily due to increased levels of operating expenses to expand our fulfillment capacity and spending on technology infrastructure and marketing efforts, partially offset by increased unit sales, including sales by marketplace sellers. 

The increase in AWS segment operating income in absolute dollars in 2015 , compared to the comparable prior year period, is primarily due to increased customer usage and cost structure productivity, partially offset by pricing changes and increased spending on technology infrastructure, which was primarily driven by additional investments to support the business growth. The decrease in AWS segment operating income in absolute dollars in 2014, compared to the comparable prior year period, is primarily due to pricing changes and increased spending on technology infrastructure, which was primarily driven by increased usage. There was a favorable impact from foreign exchange rates of $264 million, $41 million, and $38 million for 2015 , 2014 , and 2013 . 

Supplemental Information 

We expect our cost of shipping to continue to increase to the extent our customers accept and use our shipping offers at an increasing rate, our product mix shifts to the electronics and other general merchandise category, we reduce shipping rates, we use more expensive shipping methods, and we offer additional services. We seek to mitigate costs of shipping over time in part through achieving higher sales volumes, optimizing placement of fulfillment centers, negotiating better terms with our suppliers, and achieving better operating efficiencies. We believe that offering low prices to our customers is fundamental to our future success, and