I kinda think ass is way overrated. Everyone's got one, making it something straights have in common with homosexuals.
That's like fantasizing about cleavage when you have Alen-sized man-boobs.
Then again, what would a black man be in this world without a heavy bassline and a fat ass...
It's a mix. Cable companies can tell some viewership. But mostly it's done through sample polling. The polling can be passive, such as people who live in different cities and have different family demographics as representative of the overall population who have TVs equipped with systems and track and record what they're watching, "black box"-like. Mixed with them are others who are actively polled, such as people who are selected to keep diaries or are phone surveyed.
Partly... or even mostly by today's standards.