One of the problems scientists have in evaluating the placebo effect is the distinction that can be drawn between ‘true’ and ‘perceived’ placebo effects, where a ‘perceived’ effect might not not be directly attributable to the administration of a placebo. But they also acknowledge the general ignorance of definition and description when it comes to the ‘true’ placebo effect.’A review of the literature shows that most authors confuse the perceived placebo effect with the true placebo effect. The true placebo effect is highly variable, depending on several factors that are not fully understood.’

Ref. E. Ernst, K. L. Resch, Concept of true and perceived placebo effects, British Medical Journal, 1995;311:551-553