open purchase orders are generally cancellable in full or in part through the contractual provisions. 

On average, our high inventory velocity means we generally collect from consumers before our payments to suppliers come due. Inventory turnover was 9 , 9, and 10 for 2013 , 2012 , and 2011. We expect variability in inventory turnover over time since it is affected by several factors, including our product mix, the mix of sales by us and by other sellers, our continuing focus on in-stock inventory availability and selection of product offerings, our investment in new geographies and product lines, and the extent to which we choose to utilize outsource fulfillment providers. 

We believe that cash flows generated from operations and our cash, cash equivalents, and marketable securities balances will be sufficient to meet our anticipated operating cash needs for at least the next 12 months. However, any projections of future cash needs and cash flows are subject to substantial uncertainty. See Item 1A of Part I, “Risk Factors.” We continually evaluate opportunities to sell additional equity or debt securities, obtain credit facilities, repurchase common stock, pay dividends, or repurchase, refinance, or otherwise restructure our debt for strategic reasons or to further strengthen our financial position. The sale of additional equity or convertible debt securities would likely be dilutive to our shareholders. In addition, we will, from time to time, consider the acquisition of, or investment in, complementary businesses, products, services, and technologies, which might affect our liquidity requirements or cause us to issue additional equity or debt securities. There can be no assurance that additional lines-of-credit or financing instruments will be available in amounts or on terms acceptable to us, if at all. 

Results of Operations 

We have organized our operations into two segments: North America and International. We present our segment information along the same lines that our Chief Executive Officer reviews our operating results in assessing performance and allocating resources. 

Net Sales 

Net sales include product and services sales. Product sales represent revenue from the sale of products and related shipping fees and digital content where we are the seller of record. Services sales represent third-party seller fees earned (including commissions) and related shipping fees, digital content subscriptions, and non-retail activities such as AWS, advertising services, and our co-branded credit card agreements. Amazon Prime membership fees are allocated between product sales and services sales. 

Sales grew 22% , 27% , and 41% in 2013 , 2012 , and 2011 , compared to the comparable prior year periods. Changes in currency exchange rates impacted net sales by $(1.3) billion , $(854) million , and $1.1 billion for 2013 , 2012 , and 2011 . For a discussion of the effect on sales growth of exchange rates, see “Effect of Exchange Rates” below. 

North America sales grew 28% , 30% , and 43% in 2013 , 2012 , and 2011 , compared to the comparable prior year periods. The sales growth in each year primarily reflects increased unit sales, including