"Quality Change" is the new feature which will be added to the Adwords ad delivery system. As conveyed by Danny Sullivan, the quality of the search result page will vary based on the commercial nature of the keyword phrase. This means that if the keyword phrase is more non-commercial in it's origin, the number of ads displayed will be less. While if a keyword phrase is less commercial, the broad match criteria which displays ads will be used in lesser number (which tends to bring up to more ad inventory).
But the main point to notice here is that it affects the impressions for ads due to which your CTR and your quality score will be affected. This would ultimately result your ads rank in Google. Not only this it would be a source for much "qualified" leads for the advertisers . Till date Yahoo! leads the game by displaying more ads. As far as Google is concerned they have probably lessened the number of ads because now they stress on more relevant ads. Let's see how far does both the search engines succee in their plans.
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