Prescription<unk> s used in research and practice.<unk> es are a way of testing a medical therapy in which, in addition to a group of subjects that receives the treatment to be evaluated, a separate control group receives a sham<unk> " treatment which is specifically designed to have no real effect.<unk> s are most commonly used in blinded trials, where subjects do not know whether they are receiving real or<unk>  treatment. Often, there is also a further "natural history" group that does not receive any treatment at all. The purpose of the<unk>  group is to account for the<unk>  effect, that is, effects from treatment that do not depend on the treatment itself. Such factors include knowing one is receiving a treatment, attention from health care professionals, and the expectations of a treatment's effectiveness by those running the research study. Without a<unk>  group to compare against, it is not possible to know whether the treatment itself had any effect. Patients frequently show improvement even when given a sham or "fake" treatment. Such intentionally inert<unk>  treatments can take many forms, such as a pill containing only sugar, a surgery where nothing efficacious is actually done (just an incision and sometimes some minor touching or handling of the underlying structures), or a medical device (such as an ultrasound machine) that is not actually turned on. Also, due to the body's natural healing ability and statistical effects such as regression to the mean, many patients will get better even when given no treatment at all. Thus, the relevant question when assessing a treatment is not "does the treatment work?" but "does the treatment work better than a<unk>  treatment, or no treatment at all?" As one early clinical trial researcher wrote, "the first object of a therapeutic trial is to discover whether the patients who receive the treatment under investigation are cured more rapidly, more completely or more frequently, than they would have been without it." More broadly, the aim of a clinical trial is to determine what treatments, delivered in what circumstances, to which patients, in what conditions, are the most effective. Therefore, the use of<unk> s is a standard control component of most clinical trials, which attempt to make some sort of quantitative assessment of the efficacy of medicinal drugs or treatments. Such a test or clinical trial is called a<unk> , and its control is of the negative type. A study whose control is a previously tested treatment, rather than no treatment, is called a positive-control study, because its control is of the positive type. Government regulatory agencies approve new drugs only after tests establish not only that patients respond to them, but also that their effect is greater than that of a<unk>  (by way of affecting more patients, by affecting responders more strongly, or both). This close association of<unk>  effects with RCTs has a profound impact on how<unk>  effects are understood and valued in the scientific community.
Placebo-controlled study
Natural history study
Randomized controlled trial
Multicenter trial
Multiple baseline design
Prospective cohort study
Clinical study design
Retrospective cohort study