The improvement seen is also called the placebo response, and factors influencing it are also known to include hope, expectation, trust in the therapist though there may be a plethora of presently unknown contributory factors. Some early therapies which we now know to be useless, such as bloodletting for a variety of conditions, where considered efficacious at the time, due to placebo effects of the apparent treatment. Investigators take this effect into account by including a 'placebo control' group as part of their trials. Only when a treatment shows an effect on symptoms over and above the placebo effect is the treatment considered to be efficacious for the illness. One piece of jargon that has great capacity to confuse, however, is the difference between effectiveness and efficacy.
Effectiveness relates to whether a 'treatment package' works in practice.
For example, a study in which patients were given a herbal extract every week for 8 weeks, and which found that quality of life significantly improved would be justified in concluding that, at least on the basis of this study, 'the action of giving patients with a certain condition a herbal extract as part of a trial while all the hope, expectation and excitement involved' is effective in improving quality of life. What cannot be concluded is that the improvements in quality of life were caused specifically by the herbal extract i.e. that the specific efficacy of the extract had been shown. To show that, a randomised controlled trial, containing a control group taking a placebo extract, is required.