Control Group

The group that does NOT get the change and continues with the existing experience — variant A, the status quo, the baseline. It is the “what would have happened anyway” yardstick.

Without a control group you cannot tell whether an improvement came from your change or from something happening to everyone (a holiday, a marketing campaign, the weather). The whole logic of experiments is that treatment and control live through the same outside world, so subtracting one from the other cancels out everything except your change.