aristotle - 6:21 pm on May 8, 2011 (gmt 0)
What's happening with this still isn't clear to me. Originally I thought that Panda was a revision of the main search algorithm, which is supposedly updated almost continuosly.
Now it appears that Panda may involve a separate calculation whose results are fed into the main algorithm. But this separate calculation is only done periodically. Not sure if "periodically" means weekly, monthly, or some irregular interval.
The problem with a long re-run interval is that a website can be completely revamped in the meantime. For example, suppose Panda gives a certain website a "high-quality" rating. But then the next day a new owner takes it over and fills it with a lot of low-quality biased articles full of falsehoods and mis-information about a controversial social issue. Despite the addition of these new articles, the site would continue to get high rankings until the next Panda re-run, whenever that might occur.
So it seems to me that Google would need to do the Panda re-runs fairly often to avoid this kind of problem.