Last week, Netflix acquired exclusive rights to Virunga, a documentary film about a national park in eastern Congo and the people who fight to keep it alive. As The New York Times reported, the deal was part of a push by the streaming video service to add more cause-related documentaries to its programming lineup. It was also an exciting opportunity for the team behind the film, who want to reach the widest possible viewership. “The potential audience base is over 50 million subscribers and counting,” Joanna Natasegara, Virunga’s producer, wrote in an email to CJR.

That’s a dizzying figure for a nonprofit documentary that traditionally would have had a limited audience. And as the economics of theatrical release are as challenging as ever for small films, and Netflix continues its investment in documentaries, online distribution looks increasingly attractive.

There’s a catch, though. Natasegara will never know exactly how many of those 50 million subscribers actually watch Virunga, because Netflix doesn’t reveal detailed audience numbers—not to the media, not to the public, and not to the filmmakers themselves. In a data-centric age where digital media outlets meticulously count clicks while pouring time and money into understanding audiences, and journalists debate to what extent news coverage should be shaped by all this information, documentary-makers who rely on digital platforms have access to only a drop of the accumulated data.

For many filmmakers, the potential reach far outweighs the lack of data transparency—and without access to hard numbers, some experiment with alternative ways of evaluating their audience. Keri Pickett is the director of The Fabulous Ice Age, a documentary about the history of figure skating, released in February as a Netflix original. Every morning, she checks her reviews on the service—to gauge how people like her film, but also how many are watching it. “I’ve asked everybody I know who uses Netflix if they rate their film,” she says. “In my informal poll between 10-20% rate films, so I can assume that 50,000 people have seen my film.”

“I’m interested to know for sure and if I could know, I would like to know,” Pickett adds. But she says she can live without the audience numbers, and adds that we are seeing “a generational shift in how your film is viewed as a success.”

Traditionally, a documentary begins with screenings at film festivals, followed by a theatrical release and/or TV showings and DVD releases. But many films do not make it as far as the cinemas—and even for the ones that do, “it’s a more risky model,” Pickett says. “If you don’t get that instant hit, people go: ‘Oh gosh, the film isn’t that good.’”

When a filmmaker enters a deal with Netflix, the company pays a license fee to carry the film on its streaming platform for a certain number of years. That’s one reason why filmmakers get little audience data—they’re not paid by the sale, or by the viewing. But what matters most to Pickett is that with Netflix a potential, global audience will have access to her film over a long time period.

Cecilia Peck and Inbal Lessner, the filmmakers behind the Netflix original Brave Miss World—about a former Miss Israel who uses her own experiences to fight sexual assault—also take alternative approaches to understand their success. They engage with their audience on social media and through their website, where they ask survivors to share their own stories of sexual assault. “We know people are watching because they tell us on Twitter,” Lessner said. “We feel that unlike traditional broadcasters, we have the ability to develop a word of mouth following.”

While they don’t have hard numbers, executives at Netflix have given them a rough idea of how their film has been doing, Peck said. And “because we set up our own model to track our audience, [the lack of data] didn’t affect us so much. We would really recommend that to other filmmakers.”

But others in the documentary world say a rough idea is not enough, and argue that digital platforms—not only Netflix—should share more of the information they’re collecting.

“I think it’s a common complaint about all digital releases, that there’s not much transparency,” said Doug Block, an independent documentary director and producer, and the founder of The D-Word, an online community for documentary professionals.

The lack of insight into audiences makes it hard for filmmakers to promote their work to future investors, he said. And in the case of Netflix, Block is skeptical of the license-fee model: “You don’t get any extra besides your acquisition fee, whether a million people rent it or click on it. It’s downright un-American.”

Un-American or not, it’s Netflix’s business model. By analyzing subscriber preferences and activity, the company builds knowledge that informs how much it can pay for content.

“We pay what we think a piece is worth based on the audience. We look at likelihood of how big the audience is and look at cost per hour of one piece of content versus another piece of content,”Jonathan Friedland, a Netflix spokesman, said. “We don’t give that data to anybody, because if we did then we wouldn’t have the info we need to make good business decisions.”

Releasing audience data to filmmakers but not the general public is not an option, Friedland said: “In a perfect world, yes, but none of us will do that because that basically gives away the essence of what we do and how we do it… I’m very sympathetic to this notion but it isn’t very realistic.” People are welcome to distribute their content on other platforms, like iTunes, if they prefer a transaction-based model, he said.

On that platform, filmmakers get a sales report that breaks down the number of sales and rentals, and they’re paid accordingly. But transaction-based platforms can generate plenty of other data about consumer behavior that’s not available to filmmakers, too.

Lene Bech Sillesen is a CJR Delacorte Fellow. Follow her on Twitter at @LeneBechS.