tedster - 7:28 pm on Apr 16, 2011 (gmt 0)
You look for outliers...
Exactly - Matt Cutts calls them "edge cases". And with Panda, that's what Google asked the webmasters for within a week, with this thread on their Webmaster Help Forum [google.com].
It's also possible that eHow was an outlier in the other direction with Panda 1 - and the tweaks they made for Panda 2 are the reason eHow then took a hit.