We got an interesting insight into how often Google modifies their search algorithm. Google CEO Eric Schmidt told Congress that Google made 516 updates in 2010. The real shocker? They tested over 13,000 updates.
His statement was
To give you a sense of the scale of the changes that Google considers, in 2010 we conducted 13,311 precision evaluations to see whether proposed algorithm changes improved the quality of its search results, 8,157 side-by-side experiments where it presented two sets of search results to a panel of human testers and had the evaluators rank which set of results was better, and 2,800 click evaluations to see how a small sample of real-life Google users responded to the change.
Ultimately, the process resulted in 516 changes that were determined to be useful to users based on the data and, therefore, were made to Google’s algorithm. Most of these changes are imperceptible to users and affect a very small percentage of websites, but each one of them is implemented only if we believe the change will benefit our users.
The whole testimony used to be here http://judiciary.senate.gov/pdf/11-9-