It will vary from client to client. If you’re testing website content, Google has some webmaster tools that allow you to put little bits of code on your website. That code allows for dynamic content on the site. Basically you have two sets of copy stored in that database and the code goes and grabs one set when one visitor comes, and then when another visitor comes, it goes and grabs the other set and then it tracks the results about how those two visitors respond to that content, that headline, whatever it is. Google’s not the only one that can do this, but it’s free and easily accessible to most clients.