managing our operating costs, partially offset by investments we make in longer-term strategic initiatives. To increase sales of products and services, we focus on improving all aspects of the customer experience, including lowering prices, improving availability, offering faster delivery and performance times, increasing selection, increasing product categories and service offerings, expanding product information, improving ease of use, improving reliability, and earning customer trust. We also seek to efficiently manage shareholder dilution while maintaining the flexibility to issue shares for strategic purposes, such as financings, acquisitions, and aligning employee compensation with shareholders’ interests. We utilize restricted stock units as our primary vehicle for equity compensation because we believe they align the long-term interests of our shareholders and employees. In measuring shareholder dilution, we include all vested and unvested stock awards outstanding, without regard to estimated forfeitures. Total shares outstanding plus outstanding stock awards were 476 million and 470 million as of December 31, 2013 and 2012. 

We seek to reduce our variable costs per unit and work to leverage our fixed costs. Our variable costs include product and content costs, payment processing and related transaction costs, picking, packaging, and preparing orders for shipment, transportation, customer service support, costs necessary to run AWS, and a portion of our marketing costs. Our fixed costs include the costs necessary to run our technology infrastructure; to build, enhance, and add features to our websites and web services, our electronic devices, and digital offerings; and to build and optimize our fulfillment centers. Variable costs generally change directly with sales volume, while fixed costs generally increase depending on the timing of capacity needs, geographic expansion, category expansion, and other factors. To decrease our variable costs on a per unit basis and enable us to lower prices for customers, we seek to increase our direct sourcing, increase discounts available to us from suppliers, and reduce defects in our processes. To minimize growth in fixed costs, we seek to improve process efficiencies and maintain a lean culture. 

Because of our model we are able to turn our inventory quickly and have a cash-generating operating cycle. 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. Accounts payable days were 74, 76, and 74 for 2013, 2012, and 2011 . We expect some variability in accounts payable days over time since they are affected by several factors, including the mix of product sales, the mix of sales