When you're using an AB test to understand the effect of a treatment, there are a lot of assumptions about how the treatment (and control, for that matter) get applied. For example, it's easy to think that everyone who was assigned to the treatment arm actually gets the treatment, everyone in the control arm doesn't, and that the two groups get their treatment instantaneously. None of these things happen in real life, and if you really care about measuring your treatment effect then that's something you want to understand and correct. In this post we'll talk through a great blog post that outlines this for mobile experiments. Oh, and Ben sings.