How do they take TV show viewing ratings?

Question

I was looking at Wikipedia at some TV shows like doctor who and it has listed the amount of people that watched the show. I was just wondering how it is they decide this as I'm pretty sure there is no way for a TV to send a signal back to the transmitter saying what it's watching.

I was told that it's just a sample, but I don't see how they can get such accurate numbers in this way.

Answer 1

There is a group called Nielson ratings that actually has transmitters in various households.  It determines what people are watching.

Answer 2

Yes Nielsen ratings are a sampling of households and thats how they get the numbers. I once had a Nielsen box on my TV. You also have to keep a diary of what you watch. That was many years ago so they probably have a better system now. They also call around and do surveys.