How reliable are these double blind studies? How do they know they are guaranteeing that only caffeine is impacting their abilities at that moment? There could

How reliable are these double blind studies? How do they know they are guaranteeing that only caffeine is impacting their abilities at that moment? There could be a ton of other factors that could be playing a part here. Can we be sure that x number of users is enough to remove those extraneous factors? These kind of studies feel very odd.