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Amazon's A to Z shopping experience, embodied in its logo, has converted many users to Prime subscribers. Source: Amazon.com.

Amazon.com (NASDAQ:AMZN) has always been a black box when discussing its Amazon Prime subscription-based bundle of services. The company doesn't give insight into revenue or overall profitability and doesn't even report total subscribers, instead only choosing to use the phrase "tens of millions" when discussing Amazon Prime's membership size.

In the absence of direct reports, investors are forced to use third-party estimates for guidance. One of the more prominent analyst firms, Consumer Intelligence Partners, or CIRP, estimates Prime's reach based on survey results. According to CIRP (thanks to Business Insider), Amazon Prime has 54 million U.S. members as of the fourth quarter of 2015 and is still growing briskly by adding 14 million – or 35%-- in the past year alone.

If CIRPs survey is correct, it brings both positive and negative news for the retailer.

Slowing growth and slowing member spend
In a world of 2%-3% GDP growth, 35% subscription year-on-year growth is amazing. Although the rate of growth has slowed from last year's torrid 54% clip, that is to be expected, as the number of subscribers increases. However, when comparing 2015's total subscriber growth on a raw-number basis, the 14 million figure ties the prior year's addition. Overall, it appears the slowing growth rate is due to a larger base (or divisor) more so than falling demand for the product. In the end, I feel Amazon investors should feel positive about subscriber growth.

It's not all positive; the more troubling trend of CIRP's survey is Prime member spend. Amazon is able to bundle these free services -- shipping, video, and cloud storage, among others -- because, in addition to the annual fee, Prime members spend more through the company's core retailing business. According to CIRP, that figure is down rather precipitously from 2014, falling from $1,500 to $1,100 per year. Overall, CIRP reports the average non-Prime consumer spent less on the website as well, falling from $625 annually in 2014 to $600 last year.

CIRP's spending data conflicts with e-commerce trend
It should be noted the spending figure CIRP gives seems to fly in the face of the prevailing narrative regarding e-commerce and mobile commerce. For the fourth fiscal quarter, comScore predicted a 14% rise in digital commerce, $70 billion in the holiday season alone, with Amazon taking a massive 20% market share.

For Cyber Monday, the firm estimated a $3.1 billion total in digital commerce sales, up 21% from the prior year, with Amazon again taking the top spot over primarily bricks-and-sticks retailer Wal-Mart (NYSE:WMT). In anticipation of a further shift to e-commerce, Wal-Mart has cut back its storefront footprint by closing its Walmart Express store concept while spending to build out its Internet operations.

A particularly encouraging niche for Wal-Mart is a wrinkle in the traditional digital shopping experience called click-and-collect. Consumers place an order online but pick up the products in the physical store. A recent study from the International Council of Shopping Centers found one-third of shoppers used click-and-collect last holiday season, with market research firm InfoScout estimating nearly half of Americans trying the hybrid experience for the first time last year.

Could sample size be an issue?
While it's possible Amazon Prime members could have spent less money, as a 35% addition in subscribers can change the composition and attributes of a sample, a 27% drop in total spend from Amazon Prime members still seems improbable versus the broad trend toward e-commerce.

Equally improbable is the figure that non-Prime users spent less money on the site last year, as this presumably larger pool should align more closely with total digital-commerce growth. CIRP's survey is based on 500 U.S. users, and perhaps this sample isn't indicative of the entire population of U.S. Amazon Prime members and total Amazon shoppers, respectively. In all fairness to CIRP, extracting large-scale data from surveys is more art than science, and the company should be commended for its attempt to do so.

Amazon could put this debate to rest if it decided to share these figures. Sharing more information about the company's Amazon Web Services division led to huge gains, and Amazon Prime could potentially do the same.

Jamal Carnette has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Amazon.com. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.