That is/was a problem with Nielsen's diaries. People write down shows that are 'popular' but they never watch it.
Next, the person may change his viewing habits with the Nielsen box in the room. They may like a particular show, and although they don't feel like watching it (it's a repeat, a better, one time movie is on HBO, etc), they will either watch it (or at least leave the TV on so the box records it) or claim they watched it.
That's possible, but Nielsen does meter every television set within a HH.
Perhaps Nielson should start doing random quizzes to see if people actually watched! ;)
susie