Amazon isn't likely to release granular data about its Prime subscribers any time soon, which means a privately run survey has a little more weight than we might otherwise give it—especially with a conclusion as resounding as the one drawn by Consumer Intelligence Research Partners on Tuesday. The firm's latest report estimated an Amazon Prime retention rate that may only be rivaled by alcohol, cigarettes, and other drugs: 96 percent.

That's how many Prime members elect to renew the service after two full years of use, according to CIRP's estimate (which involved a survey of 2,108 people in the US). CIRP's numbers also found that 91 percent of one-year subscribers elect to renew for a second Prime year, while 73 percent of the service's free-trial users decide to pay for Prime.

The survey tracked renewal rates in all three of those cases, dating back as far as the second fiscal quarter of 2014, and it found that 30-day trial upgraders slowed down from June of 2014 until September of last year—which CIRP's study authors attributed to the March 2014 bump in Prime fees from $79 to $99. The price of Prime in the UK also increased from £49 to £79 around the same time. The rates slowly climbed from that point on, and they've had an upward trend in the other categories, as well—one-year renewals are up from 81 percent in early 2014, and two-year renewals have climbed from 81 percent that year, as well.

"Customers that join on a 30-day free trial base the decision in part on the free trial experience, and mostly on their expectation of using Amazon Prime over a full year," CIRP co-founder Josh Lowitz said about the disparity between renewal times. "Customers that finish the first year have more data, and decide whether to pay for a subsequent year based on how much they actually used Amazon Prime during their first full year." The CIRP report also noted the rapidly increasing number of benefits Prime customers have gotten over the years, including access to exclusive TV series and films and free one-day (and in some markets, two-hour) shipping upgrades. The survey failed to mention the service's rising trend of blocking non-Prime customers from buying certain products.

Amazon doesn't disclose member numbers, but CIRP has also tried to fill in that data gap; its last major Amazon-related survey estimated a whopping 54 million Prime subscribers in the US as of December 31, 2015.