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Adwords in adsense?
Split testing is a simple idea and with channels in adsense you can split test that.
With adwords you will need different landing pages and a way of tracking which ad produced which sale or lack of sale.
e.g. target keyword is the same but you change the title on each ad. Then after a week you check the stats to see which had the highest clickthrough.
I presume this is possible some how?
How many people actually bother to do this?
A lot of people test ROI based on both ads and keywords. These are often the same group of people who base ROI/bottom line profits off of ad position and often have ads where they try to maintain a specific position as well as price range.