It is one of the most elementary tenets of scientific research that a well-designed experiment should only contain one variable. if you're testing for the effect of a particular substance you have to be sure that its presence or absence is the only thing that changes between tests. This is particularly important when you're testing humans, those multi-channel receivers of stimuli, obvious and subliminal. it's no good testing a tranquilliser and having a happy, smiley set of lab technicians administer the real thing while surly, sour-faced assistants chuck the placebos at the subjects because you won't be able to tell what's responsible for your results. And this fact can make things very tricky when the presence of the substance you're testing is inextricably bound up with the absence of a different but equally potent...
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