Comment by margor

Nielsen is global Company that is used for pricing ads on TV stations around the world. Their way of doing it from what I know personally is that they sign up people to put this ACR boxes voluntarily and then based on extrapolation of data - they can tell you pretty close what was the viewership across demographics. Similar to how election polls are done - you just need a good sample.

It's been done for at least good 15 years.