The placebo effect has been known since antiquity : it is the positive effect that can be observed after any therapeutic act, independently of the effectiveness of the act itself. For example, to treat pain, the administration of a perfectly neutral substance such as sugar or water causes an improvement. This is not due to the effect of the substance itself, but to various non-specific psycho-physiological effects induced by the treatment. This effect has been widely observed in medicine, on subjective manifestations such as nausea or pain, but also on observable clinical and biological signs  (blood pressure, dopamine release in the brain, etc.). What we do know is that the placebo effect is attributable, at least in part, to the care situation itself, to the context. The placebo effect thus varies according to the relationship between the doctor and the patient, or according to the patient's expectations . For example, the placebo effect is greater if the treatment is known to be expensive . In any case, it is a very robust effect, observed not only in patients awaiting treatment but also in healthy volunteers with no particular problem .
Conclusion: even if a device has no efficacy of its own, it can have a positive effect on the person, simply because of the therapeutic situation that surrounds it. For this reason, if we wish to determine the efficacy of a device, it is essential to compare its effects with those produced by a device that is known to be neutral, but which is offered in the same situation as the treatment. This neutral treatment is called a placebo, and is referred to as a controlled trial, because the 'test' group is compared with a control group that receives the placebo.
 Patrick Lemoine, Le mystère du placebo (1996) aux éditions Odile Jacob
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