Amazon doesn’t say how many Prime members it has, but a new estimate pegs the U.S. number at 54 million. But average spending per member declines with growth.

Amazon.com Inc. grew its Prime membership roll 35% year over year, closing 2015 with 54 million U.S. members, according to a new estimate from Consumer Intelligence Research Partners LLC, a securities research firm. That’s equal to 47% of all U.S. Amazon shoppers, the firm says.

CIRP bases its findings on a survey it runs quarterly of 500 U.S. Amazon shoppers who made a purchase on Amazon.com during the quarter. In the October to December period in 2014 CIRP estimated Amazon had 40 million Prime members.

Seven million U.S. consumers, or half of the total yearly membership gain of 14 million, joined Prime during the fourth quarter, CIRP says. Amazon does not release Prime totals but said in December it added 3 million Prime members in the third week of December alone.

However, as the number of Prime members increased, the average spending per Prime member fell. In Q4 2014, CIRP estimated Prime members spent an average of $1,500 yearly on Amazon. In Q4 2015, that dropped 27% to $1,100. Michael Levin, CIRP co-founder and partner, sees two reasons for the change. He says free streaming media options are becoming a bigger part of Prime, and some Prime members paying the $99 annual fee prioritize that over the expedited shipping benefit, leading to lower purchasing on Amazon.com. “Second, as Amazon Prime grows, it starts to include a wider range of consumers, including casual or less frequent shoppers,” he says, noting that early Prime members were the biggest spenders.

Spending on Amazon by non-Prime customers changed only slightly, to $600 in Q4 2015 from $625 in Q4 2014.

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CIRP’s Prime estimate is the highest of those released by investment analysts over the past month. Investment bankers at Cowen & Co. estimated U.S. Prime membership in December at nearly 41 million, based on a monthly survey it conducts of 2,500 U.S. consumers. It said Amazon added 10 million members to Prime between December 2014 and December 2015.

Meanwhile Macquarie Research investment analysts issued what it called a “likely quite conservative” estimate in December that 25% of U.S. households included a Prime member in December. It projects 50% of U.S. households will have Prime memberships in 2020.

Macquarie estimates that Amazon took 24 cents of every $1 of retail sales growth in 2015. In the e-retail segment, Amazon’s dominance was even more commanding, with 51 cents of every $1 in sales growth going to the company. It further estimated that Amazon’s gross U.S. sales grew by $22 billion, or 33%, to $88 billion in 2015 from $66 billion in 2014, including sales by Amazon and by other merchants selling on Amazon.com.

Industry watchers will be monitoring how such estimates pan out when Amazon reports its Q4 financials Thursday. Amazon is the No. 1 e-retailer in North America and Europe, as ranked in Internet Retailer’s Top 500 and Europe 500 research guides.

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