Which audience analysis measurements principles and techniques are most effective, and why?

There are many different audience analysis measurements principles and techniques.  These techniques are used to determine how many people are tuning into whatever type of media and for how long, or what they are paying for to view or read.  Newspapers and magazines count how much of their product is in circulation; companies can easily track subscriptions and also how much of their product was bought in stores by counting what wasn't sold.  I think this technique is extremely effective and practical.  There are other types of techniques that are less effective, like broadcast viewing which counts how long people tune into specific programs.  The reason I feel that this is less attractive is because there are so many viewers to keep track of while watching TV or listening to the radio and they often change channels many times between.  A good way to show what people are watching is by the ratings, so if a lot of people are tuning into a program you will know that it is effective and is also a good place for advertising.  Although measuring audiences for mass media is a daunting task it needs to be done in order to help the advertising companies grow and make money, they need to know where their best advertising opportunities are if they want to be effective.