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.