From direct mail to web design, A/B testing is considered a gold standard of user research: Show one version to half your audience and another version to the other half; compare results, and adjust accordingly. Some very cool examples include Google’s obsessive testing of subtle design tweaks and Dustin Curtis’ experiment with direct commands and clickthrough rates. (“You should follow me on Twitter” produced dramatically better results than the less moralizing, “Follow me on Twitter.”)
So here’s something devilishly brilliant: The Huffington Post applies A/B testing to some of its headlines. Readers are randomly shown one of two headlines for the same story. After five minutes, which is enough time for such a high-traffic site, the version with the most clicks becomes the wood that everyone sees.
(via How The Huffington Post uses real-time testing to write better headlines – niemanlab.org)
I also found it interesting that they are considering splitting up the content they serve by IP address, so they can serve the East and West coasts better.