This is why SEO companies stay in business :)
"A typical algorithmic change begins as an idea from one of our engineers. We then implement that idea on a test version of Google and generate before and after results pages. "
"We take a data-driven approach and employ analysts, researchers and statisticians to evaluate search quality on a full-time basis. Changes to our algorithms undergo extensive quality evaluation before being released."
Amazing Explanation page on how search works - http://www.google.com/competition/howgooglesearchworks.html