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.