The U.S. Commerce Department reports monthly on the health of the retail sector. These numbers, based on surveys of 12,000 retail businesses conducted each month, are closely watched by retailers, analysts and investors, as they are the most comprehensive figures available on U.S. retail sales as a whole and by product category.

As part of that monthly report, the agency discloses its estimates of what it calls nonstore sales. Nonstore sales mainly take place online, but also include other sales that take place outside of stores, including mail and telephone orders, door-to-door sales and sales through vending machines.

Internet Retailer uses the monthly nonstore figures disclosed by the Commerce Department as an early indicator of the health of the e-commerce market, on which the agency reports sales on a quarterly basis.

A historical look at the correlation between nonstore sales and e-commerce sales shows that e-commerce is a growing portion of these sales. In the second quarter of 2017, for example, the latest quarter for which data is available, e-commerce represented 72.5% of nonstore sales. That compares with 68.8% in the second quarter of 2016.