Any idea how companies like Nielsen get their stats?
It was very simple. For some period of time (I think it was a week...this was a long time ago so details are fuzzy) I was supposed to write down the time and identity of every radio station I heard (or maybe every station I heard for more than a few minutes?). At the end of the survey period, I mailed that to them.
I forget how much they paid, but I think it was on the order of a few dollars.
This would have been quite a while ago--probably at least 20 years ago.
They still do that kind of survey, I believe, a few times a year in each of several hundred metropolitan areas.
In 2007, they added another data gathering method that they use in a smaller number of major markets where participants wear a small device that can pick up subaudible identification messages embedded in radio broadcasts to tell what radio stations are being played in an area. These devices record that information and periodically send the data back to the company.
In 2012 they were bought by Neilsen and are now called Neilsen Audio, but the data gathering is still as described above.
TV-tracking is done by sound signatures and web-tracking is done with a router which has custom firmware installed.