I have always wondered how does the television networks know how many are watching their program? For instance; American Idol. They announce so many millions are watching tonight. How do they know that? Do they know when I change the channel?

How does a radio program like Coast to Coast know they have millions of listeners tonight.

This has always stumped me.

Its called Neilsons ratings and you can find out about them here:

Nielsen Media Research (http://www.nielsenmedia.com)