The idea is to test the effect of the specific drug, not the effect of administering… anything. The placebo is used to make the experience of the control group (i.e. the group not getting the drug) exactly the same as the experience of the group getting the drug; the only difference is the presence/absence of the drug itself. That makes the comparison a true test of the drug. Because the subjects don’t know which group they’re in, this is called a “blind” trial.
If the people conducting the trial know if a particular subject is getting the drug or the placebo, this might influence their behaviour when conducting the trial. (They might behave differently when administering the drug/placebo, or their knowledge might influence how they interpret the clinical results.) For this reason, the information about which subjects get the drug and which get the placebo should be hidden from the people conducting the trial until the results are finally analysed. Because both the subjects and researchers don’t know who’s in which group, this is called a “double-blind” trial.
Like all good experimental science, the whole idea is to design experiments that stop us fooling ourselves by our preconceptions, and instead to find out what’s really going on.
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