Last week was the standard fixed-horizon test: split traffic 50/50, wait for your pre-calculated sample size, look once, done. That's still the right default most of the time. It's not the only tool, though, and treating it as the only one gets expensive once its assumptions stop matching your situation. Why the default isn't always right A fixed-horizon test answers one question: is there a detectable difference between two fixed groups, at a preset error tolerance, once enough ...