Ethical issues arise
There are a few practicalities that complicate experimentation with human subjects. For ethical reasons, experiments involving potential danger to the subjects are not possible. Even if there is no known danger, the subjects should be aware of what is involved in the experiment and must give informed consent.
Controls
Treatments that involve change should usually be compared to a 'treatment' in which there is no change. Individuals for whom there has been no change are called controls.
Placebos
Unfortunately, the act of administering a treatment to a human subject may itself affect the response, irrespective of the treatment effect. For example, if a drug is being assessed for its ability to reduce headaches, the knowledge that medication has been administered may make the subject feel better, even if the drug has no active ingredient.
To avoid the psychological effect of the treatment on the subject being confounded with the effect of the drug, an indistinguishable 'treatment' with no effect may be given to the control group of subjects; this is called a placebo.
Double-blind experiments
A further complication may arise when the measured response from each subject may be affected by knowledge of the treatment applied. If the experimenter knows which treatment has been applied to each experimental unit, there may be a subconscious tendency to systematically over- or under-assess one treatment.
In a double-blind experiment, neither the experimenter not the subject knows which treatment has been applied. Again, the aim is to ensure that other factors do not act as lurking variables to confound comparisons of the treatments.