Online grocery sites get a lot of eyeballs on desktops, while fashion retailer Yoox dominates mobile site time, the web analytics company finds.

It’s tough for online retailers to keep consumers on their sites, whether because shoppers are faster at navigating pages, are highly distracted or have more sites to explore, so merchants who increase engagement are doing something right, says Pavel Tuchinsky, digital insights manager for digital analytics firm SimilarWeb.

Who’s winning at engagement? SimilarWeb has compiled an index exclusively for Internet Retailer of the Top 10 retailers from the Top 500 Guide. The index measures and ranks desktop engagement, mobile engagement and app engagement. Engagement in this context is defined by how long visitors remain on an e-retail site, mobile site or mobile app, and how many pages they view in one visit.

Good design, ease of use and promotional activity can determine how long a consumer spends on a retailer’s site, Tuchinsky says, as can the nature of the task. Grocery shopping, for example, takes time and generates longer site visits. Online grocer Peapod, No. 69 in the Internet Retailer 2015 Top 500 Guide, grabbed the top spot on average desktop visit duration, at 13:57 minutes, SimilarWeb found.

“Like No. 3 in our list, (No. 83 in the Top 500), this shows that many of us are spending time buying our groceries online for the entire week, with Peapod customers visiting 18.6 pages in a session and Fresh Direct visitors 17.1 pages,” Tuchinsky says. “ (No. 211), the top retailer for mobile web engagement, is the gold standard for mobile retail engagement. Its mobile version is stylish, with easy navigation, ordering, and returns, resulting in people shopping on mobile for more than 10 minutes per session,” Tuchinsky says.

Here are the rankings from SimilarWeb: