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Therefore if you have a page about cats and want dog ads, it's no use in just changing the meta tags. You need to change the content of the page to be about dogs. That's going to really hack off the people that came to the page about cats, and they probably will click back, not through.
Besides, chasing keywords is a mugs game. Today's hot keywords will be yesterdays by the time a new page has been constructed and drawing the traffic.
How far can I deviate for ads?
The question I'd throw back is: "How do you indend to implement the deviation?"
One section of my site is a reference on advertising art for a particular consumable product. My visitors are indeed interested in the art, however, if they were to buy something would probably be more inclined to buy items relating to the actual product and not the art or artist.
I've tried many different mechanisms to "focus" the ads toward the product and away from the art (while still adhering to the TOS and not screwing up my SERPs) but have not achieved the level of success to see any consistant earnings.
Making extensive use of section targeting can get me to exactly where I want to be but, as I can't tell the crawler to STOP... it'll wander off into yet another area of feeble targeting in a few days.
Best of luck!