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Amazon Prime gets a sign-up boost from a big day of sales

A day of hot deals in July helped drive a 7 percent jump in Amazon Prime membership last quarter, according to a market researcher.

Prime members spend more money on the site than nonmembers do, around $1,200 vs. $600 per year, says CIRP. Amazon

Amazon got a nice present last quarter for its 20th birthday.

On July 15, Amazon celebrated two decades in existence with Prime Day, a global shopping event offering special deals exclusively for Prime members. During the third quarter, US Prime membership grew by 7 percent compared with 3 percent in the same quarter last year, according to a report out Tuesday from Consumer Intelligence Research Partners.

A quarter of those polled by CIRP said they became Prime members to take advantage of the deals on Amazon Prime Day.

Offering free two-day shipping on most items with no minimum required, Amazon Prime is a key sales driver for the company. Less than 1 percent of Prime members are likely to consider buying from any of the other most popular online retailers during the same session, according to an April report from research firm Millward Brown Digital. Prime members spend more money on the site than do nonmembers, around $1,200 versus $600 per year, CIRP said.

Still, Amazon has to keep an eye on the competition. Walmart is testing a free three-day shipping program called ShippingPass for $50 a year. Earlier this month, online store eliminated its annual $50 membership fee. All of this competition is ultimately healthy for the consumer because it pressures retailers to expand their product offerings and keep prices attractive to retain existing customers and win over new ones.

Amazon continues to enhance its offerings beyond just products. In July, the retail giant expanded its Home Service hub for hiring plumbers, painters and electricians to 11 additional cities. In late September, it launched Amazon Flex, which lets people become drivers for Amazon's on-demand delivery service.

But Prime continues to be a key strategy for drawing in buyers. Amazon Prime added 3 million subscribers during the third quarter of 2015, according to CIRP, and 6 million members over the past two quarters. As of September 30, around 46 percent of all Amazon customers had a Prime subscription, estimates CIRP, which the research firm says translates into around 47 million Prime members.

Priced at $99 a year, a Prime membership offers subscribers free two-day shipping, Prime instant video, unlimited music streaming with Prime Music, unlimited photo storage with Prime Photos and a Kindle Owner's Lending Library with more than 800,000 e-books.

CIRP based its results on a poll of 500 people in the US who made a purchase at from July through September.

An Amazon spokeswoman said the company had nothing to share at this time other than its Prime Day press release from July 17.

Update, 1:20 p.m. PT: Adds comment from Amazon.