Google Analytics stopped counting bounce visits when calculating the average time on site. The new numbers began showing up on July 20th.
I finally received a satisfying answer from the Google Analytics support team, following this post. This explains the jump in the Avg. time on site metric.
Here’s a quote from their support team:
Previously, the Average Time on Site had been calculated as the total time on site for all visits divided by the total number of visits. Both the total time on site and total number of visits included bounces.
As of July 20, 2007, we began reporting the Average Time on Site as the total time on site for all visits (excluding bounces) divided by the total number of visits (excluding bounces). This change also affected data from earlier dates, not just newer data.
Because bounces have been removed from this calculation, you may notice a significant increase in the average amount of time spent on your site. Please be assured that we have in no way changed your data, only the way
that the Average Time on Site is being calculated.
I still wonder why they made this change, and also why they hadn’t reported it on their offical blog.
Anyway, it’s good knowing what this jump in numbers was all about.
Update: Google has returned to the previous policy: Bounce hits are counted as well. So now the equasion of Avg. Time on site is back to: time of all the visits divided by the total number of visits.
Thanks to Bryan for notifying me.