Amazon sells more goods than any one person could count – but the e-commerce giant’s true “core product” is convenience, and how quickly it can get an order from customers’ virtual shopping carts to their real-life doorsteps.

Part of what makes it so easy for Amazon to offer two-day or even same-day shipping to customers is its vast network of distribution centers, which are located across the U.S. and store and ship products to their final destinations. New research from Wharton business economics and public policy professor Katja Seim takes a closer look at how significantly expanding that distribution center network over the past decade has been key to Amazon’s growth strategy.

Seim recently spoke to Knowledge at Wharton about her paper, “Economies of Density in E-Commerce: A Study of Amazon’s Fulfillment Center Network,” which was co-authored with Cornell’s Jean-Francois Houde and Penn State’s Peter Newberry.

An edited transcript of the conversation follows.

Knowledge at Wharton: Your research is…