1 Answer

There are two main ways to do this. the first, and the one that has been around the longest is the Nielson rating system. basically, a certain number of homes have a box that does track what programs they watch. that data is taken and then mutliplied out to an estimate. for example, if 1/10 of the data indicates a show was watched then it is assumed that 1/10 of the view population did so as well.
the second is more accurate. if you have cable or satellite when you tune to a channel it is digital. the more people that tune to that channel, the higher the bandwith is going to be to accomadate all the users without a downgrade in reception. there are formulas to determine that a certain number of bandwith usage equals a certain number of viewers.