How many people are really watching Disney+ and Netflix?

For those obsessed with who is winning the video streaming wars, one metric matters: subscriber growth. But Netflix Inc and now Walt Disney Co - with its November launch of Disney+ - typically release that figure quarterly, leaving outsiders to guess at subscriber growth in any way they can.

A cottage industry of companies has sprung up to fill that vacuum. Firms like Apptopia, Sensor Tower and App Annie, born years ago to track how many people download mobile apps, are now playing a bigger role in the streaming war that kicks into gear this year as AT&T Inc’s WarnerMedia and Comcast Corp-owned NBCUniversal launch new services.

These firms sell mobile download data they arrive at by applying algorithmic magic to publicly available data and data from other apps. The process is propriety, they say, and opaque to outsiders.

The resulting figures - which are approximations of mobile downloads, not the new subscribers the companies disclose - do not correlate exactly with subscriber growth, but are influential.

Third-party data is widely reported in the press, including in Reuters stories. Bloomberg offers Apptopia’s mobile data to its clients. The data is also cited in research from Wall Street firms including Credit Suisse, Bank of America and Wells Fargo - sometimes as a worthwhile indication of performance, and other times dismissively.

The data moves markets: On Nov. 26, shortly after Apptopia released data indicating that Disney+ was averaging nearly a million new subscribers a day – a report that was covered widely in the press – Disney shares rose 2.3% to $153.43, setting a new record high.

To survey how often these firms get it right, Reuters reviewed eight quarters of data from Netflix, and the same amount of data from two of the third-party app measurement firms. It found that Sensor Tower’s past eight quarters’ of Netflix mobile download data has directionally if not precisely mirrored Netflix global paid membership growth. Apptopia download data mirrored it directionally in all but two quarters.