Most experimentation teams I talk to are more worried about concurrent tests than they should be. They treat isolation as the default, queue up experiments sequentially, and then wonder why velocity is low. The interaction effect problem is real in theory. In practice, it barely shows up. What Interaction Effects Are (and Why People Panic) When two A/B tests run simultaneously on the same user population, there's a risk that they interfere with each other. Test A changes the header. Test B changes the checkout button. A user sees both. The concern is that the combined effect is different from what you'd expect by adding the individual effects together. In statistics, this is called a qualitative interaction: the direction of one test's result reverses depending on the state of another. That's the scary version. What actually happens is almost never that dramatic. What the Data Actually Shows Microsoft's experimentation team ran a rigorous analysis across four...