JS_Harris - 10:04 pm on Aug 1, 2009 (gmt 0)

Google rotates different data sets into the serps depending on time. You can see if that is what's happening by looking at the TOTAL number of pages returned and compare it with your yo-yo charts. During any decrease in traffic there seems to be an increase in number of pages returned.

This also explains peoples complaint of "I was #3 and now i'm down to #12, why?" with the obligatory "I'm back at #3" when it returns to the previous data set.

My yo-yo charts suggest a different set of results (and more of them) from Friday to Sunday but it varies by site and topic.

Keeping webmasters guessing keeps Google in business, what better way to do that then to use multiple data sets? A sprinkle of chaos theory in the algorithm can skew even the most diligent of studies especially when modified over time.