How can they tell number of viewers of a certain tv show?

At the moment, they still do it as they have for decades. A research firm called Nielsen selected a whole bunch of families all across the country and these families complete a diary of what they watched that week and then they return it to Nielsen and they analyze the data and pass it on to the networks. I would assume, although I don't know for sure, that now most of the country gets either cable or satellite feeds that there would be some technology available to tell what people are watching. Of course, it would be easier to do that if every channel had a separate cable to your house; naturally it doesn't work that way. Phil