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It may seem minor, but I've seen fluctuations from up to 0.5% just based on a different display URL.
If domain is one word, split test:
I find that example.com IF it's an 'authority' word/site and IF the company specializes in one product works very well.
If it is an authority site, but doesn't meet the above criteria or isn't an authority site at all, then example.com/keyword usually works well - but still test.
An example of the above is HP.com.
HP makes laptops, computers, printers, PDAs, etc.
Often users are afraid that clicking on an HP type ad can result in a lot of confusion of where they're going. In this case, HP.com/printers often works better than HP.com as a display URL.
If domain is two words, split test:
(widgetexamples.com just doesn't cut it).
If keyword is 3 or more words:
Same as above, but often Keyword.ThreeWidgetExamples.com or ThreeWidgetExamples.com/Keyword is so long it starts to wear on the user and usually ends up with less clicks, unless it's a parody/joke type ad.
One of the reasons to test all the possible variations is that some markets respond more strongly to one over the other. For user demographics that aren't net savvy, the /keyword or keyword.domain.com usually works better. For interent savvy markets, the reverse is often true. (Not always - just often - test)
In respect to the domain, we often use a .co.uk for a UK campaign and .com for a non-UK and it has a big impact. The domain itself we've not found to be massively influential, but certainly the TLD has.
From hypothetical formula [webmasterworld.com] thread.
I think the TLD discussion an interesting perspective to bring into the URL formula as well.