In the five years since Netflix started streaming original series like the Emmy-winning "House of Cards" and "Master of None," the shows have had a question hanging over them: How many people are watching?

Outside of Netflix, nobody knows the answer.

That's because, much to the frustration of those in the industry who would like to have a firm idea of just how popular those shows are, the streaming services — Amazon and Hulu included — have been fiercely protective of their numbers.

Now, Nielsen, the 94-year-old company that for decades has had an effective monopoly on measuring television ratings in the United States, has announced that it has found a way into the great unknown of Netflix viewership.

What Nielsen's data show, exactly, and how rigorously they are measured remains something of a mystery, because Nielsen did not release the data publicly. The move, however, is a step toward finding a reliable third-party ratings system for streaming services.

"The important part of this is it provides transparency for an environment that has been pretty much a blind spot for the studios and broadcasters," said Megan Clarken, the president of Nielsen's so-called Watch division.

Nielsen announced the initiative Wednesday, but it has been collecting Netflix viewership data over the last two months in a kind of test run.

The company said it was able to determine how many viewers were streaming Netflix content through audio recognition software in the 44,000 Nielsen-rated homes across the United States.

Nielsen has been releasing its Netflix data privately to media companies that have subscribed to the service, including the Walt Disney Co., Warner Bros., Lionsgate, NBCUniversal and A&E Networks.

Nielsen executives said that those companies would now have the ability to access viewership figures for shows they licensed to Netflix, like "Friends" from Warner Bros., as well as originals like "Orange is the New Black" and "Stranger Things."

Nielsen executives said the company had not yet begun collecting data from other streaming services like Hulu or Amazon.

The move is a necessary step for Nielsen — and one that many in the industry have said is long overdue. Some entertainment executives have criticized the company as being a relic of the channel-surfing era while more viewers take to streaming content.