For years I have heard nothing good about Nielsen's sampling. We have seen recently how Nielsen completely disagrees with Hulu's own numbers. I have seen an industry exec slam Nielsen at a panel. But now I have some idea of why, even if it's from a 1992 book -- Henry Jenkins comes through yet again, in his important Textual Poachers (pp. 29-30).
As Eilieen Meehan (1990) has suggested, despite the myth of popular choice that surrounds them, the Nielsen ratings reflect only a narrowly chosen segment of the television audience--a "commodity audience"--which can be sold to national advertisers and networks, but which reflects neither mass taste nor the taste of an intellectual elite.
Given the current diversity of channels (in the broader sense, not just "television channel") compared to "the big three" era (poor PBS, never counted), this approach won't work. Is it Nielsen's current approach? I don't know. But it's food for thought.