In a Google Tech Talk on October 22, 2012, Google's Chief Economist Hal Varian said that Google runs a lot of experiments. So many that a typical Google searcher/user will see a dozen or more experiments when they access Google.
Any time you access Google, you probably are in a dozen or more experiments.
Here is the video:
The talk was about how to handle all that data and with dozens of experiments seen and acted on by each Google user several times per day - that is a lot of data.
For a webmaster and SEO - it can be a challenge managing all those variables as well.
Forum discussion at Google+.