How does Google evaluate algorithmic changes?

Today's webmaster video is about how Google evaluates algorithmic changes.

Matt answering a question "What are some of the metrics that Google uses to evaluate whether one iteration of the ranking algorithm is delivering better quality results to users than another?"

There's more about evaluation of our search results on the Official Google Blog:
http://googleblog.blogspot.com/2008/09/search-evaluation-at-google.html and on the Inside Search site:
http://www.google.com/insidesearch/howsearchworks/algorithms.html.