2. Behind the scenes: A/B testing part 2: How we test
http://37signals.com/svn/posts/2983-behind-the-scenes-ab-testing-part-2-how-we-test
3. Behind the scenes: A/B testing part 3: Finalé
http://37signals.com/svn/posts/2991-behind-the-scenes-ab-testing-part-3-final