How does Netflix and Amazon calculate ROI from their content investments?

This is has interested me for quite a while but I couldn't find any explanation of it online.

I know they know what kind of shows to make and what the audience interests are. But, how do they determine --

- how much to spend? - how often to spend?

How do they know if something they made is worth the investment? What is a success metric for them? Obviously people watching it and applauding the content and coming back for more is the first and foremost. But the end goal is two things acquiring new audience and audience retention. Are all their content investments purely for audience retention? Is there a way they can actually compute new audience acquisition or conversion rate from new content?

I am curious how they go about all of this.