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Subscribers to Amazon.com Inc.’s (NASDAQ: AMZN) Prime service spend 80% more time shopping online than do non-Prime shoppers. And not only do they spend more time at Amazon’s site, but they spend more time browsing other retail sites as well.

The data were reported by market research firm GfK, based on a tracking study the firm conducted between August and December of last year on 300 Amazon Prime members and 419 non-members, all of whom owned a smartphone.

Amazon Prime members spent an average of 12.8 hours on online shopping-related activity, some 80% more than the 7.2 hours spent by non-Prime members. Prime members’ most-used site was Amazon (78%), but more than 60% said they also visit and shop at other retailers, such as Wal-Mart Stores Inc. (NYSE: WMT) and online-only merchants.

Why do Prime members shop more? For one thing, they have higher incomes than non-Prime members. Nearly 30% of Prime members reported household income of more than $100,000, nearly double the 16% of non-Prime members with a similar income level. About 40% of Prime members reported household income between $50,000 and $99,999, compared with 33% of non-Prime members.

GfK Senior Research Director Christina Pate told eMarketer:

[Prime members] are a big portion of the online shopping population. There’s a lot of time spent shopping outside of Amazon. You can’t cut them out. Even though they are spending a lot of time on Amazon, there are a lot of opportunities to reach them.

Amazon does not report the number of Prime subscribers, but one estimate puts that number at around 50 million in the United States. That leaves more than 60 million U.S. households that remain to be signed up — a solid growth opportunity for Amazon, and good news even for its competitors.