[Tom]

People have something called a BARB box (about 5000 homes from all different walks of life) and it records viewing habits and stuff ... then they do some calculations and we get the ratings. Apparently there is only a 5% margin for error, so its supposed to be quite accurate though I'm not convinced of it.

Then 2 weeks later they release further figures with play back and without taking into account advert air time etc and these are usually a bit higher (officials)