In a move that has profound implications for the way advertisers and agencies plan and buy television, Nielsen Tuesday informed clients it will make some fundamental changes in the way it calculates its so-called "average audience" ratings - long the currency of the $80 billion TV advertising marketplace. Perhaps the most significant of the changes is that Nielsen will begin including duplicate viewing to all program telecasts in its average audience ratings, a move that could undermine one of the core tenants of Madison Avenue's media planning theory: unduplicated reach.
Nielsen said it is making the moves, effective in December, to help prepare for other big changes[...]
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In the short run, when this change is first put into place I'm guessing the change will not be significant, particularly for the Live+SD numbers that are most frequently reported. The ramifications will be much bigger potentially for the Live+7 measures, which don't really matter much (other than PR) and the C3 (commercial viewing live plus 3 days) which do matter a lot.
Hopefully at least initially Nielsen will release both the duplicated and unduplicated average numbers so the numbers are easy to compare. The move is not completely unprecedented in terms of Nielsen measurement where it already has a "gross average audience" metric (which with syndicated shows is very commonly used).
The longer-term ramifications, at least for web sites like ours will be that comparisons to past data will be less apples-to-apples than they already are.