answer:You take a sample of the population, say 10% of the total population, and try to keep the demographics of the sample group as close to the population as possible. (So there are the same percentage of children, teens, 20 to 30 years olds etc). You give them diaries to record what that watch. If half of your sample were watching the Super Bowl then you could guess that half of the total population were watching the Super Bowl. This was done by Arthur Nielsen originally on the radio in the 1920’s and 1930’s and moved to the TV in the 50’s. Of course now a days instead of the people just writing down what they are watching they have a little box attached to their TV that keeps track of it all.