Futurama: Cartoon Network's Plan

I think what they do is this (and I might be wrong, or I might even be right but it might be different in America):

They give a few select families and people some set top units for their TV sets, and a special remote. If you have the set top unit and you are watching a program, there will be a little beep or a small image in the bottom corner of the screen at the beginning of the program. To show that you are watching it, you press a button on the remote when the little image or beep comes up. It will pop up again in the middle of the program and at the end, and if you have pressed the button at all of these points in time, the set top unit will record you having watched this program. I think there are separate buttons for each person in the family, and you program your age and gender in for your own button.

Then they box reports the information back to the Nielsen headquarters. They group this information with the family's city, income and various other factors, as well as the information for the individual people who watched each program. Then they work out trends within the information they received. For example, if 50% of boys with set top units in middle class Sydney families aged 5-12 watched Yu-Gi-Oh! at 7.30 in the morning, they will say that 50% of all boys aged 5-12 in middle class Sydney families watched that program. So by having a range of people with set-top boxes, they are able to get an idea of who watched what.

The problem is, of course, that not all families are the same. Maybe saying that this amount of 12-year olds watch Buffy is not an accurate representation of 12-year olds. But it's the system as I think it works, and that's the way it stands, for now, anyway.

EDIT: Oops, I was responding to KAH's post at the bottom of page one, and I didn't realise there was another page, full of people explaining the Nielsen ratings! Sorry!

Also... my 5th TOTPD! YAAAYY!