I presume you are talking about this : http://en.wikipedia.org/wiki/A/B_testing ? My ignorance did not let me understand the question right away, i thought the link above should help others grasp the idea.
Cheers !
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Nicolas Pastorino
Director Community - eZ
Member of the Community Project Board
The quickest way of making some tests would be playing a bit with template code which would generate different outputs for designs being tested (with additional parameters being put to URL for analytics).
But surely, more advanced extension for this specific purpose would be nice.
It appears that there may be "multivariate testing" coming in the Enterprise version. I can't find anything on the eZ.no site but this article on the release on eZ publish 4.4 mentions it (at the bottom of the page)
And for Enterprise Customers, official API support, analytics and multivariate testing are in the works.