>>14794863HISTORICALLY it's only people who have Nielson boxes. However I believe that with the rise of digital, they actually are measuring much more. I'm pretty sure YouTubeTV and Hulu + Live TV are included in the ratings measurements. I think Xfinity's current cable box they give to people might measure viewership too, but I'm not 100%.
Yeah it used to be only the boxes, but they have other ways of gathering data now.