I check my ratings by the little Sitemeter logo on the right sidebar. Radio stations check ratings too. Arbitron provides standard ratings for radio stations and they're the industry standard. I've had friends in radio who told me that they lived and died by their Arbitron ratings.
I often wondered where they got their data and now I know part of the story. A couple of weeks ago, Milady was approached by the Arbitron folks. We've been participating in the survey for several days and it'll be over on Wednesday. It's been interesting keeping track of which radio station we listen to, and when we listen to it.
Arbitron does a good job of formatting their survey and the process has been fairly painless. I normally only listen to the radio while I'm driving and the radio is generally set on one of three local stations. Which station is on the dial at any given time is purely arbitrary. I'm not sure what data they'll mine from my survey, but I hope it's useful.