In the design of experiments, treatments are applied to experimental units in the treatment group(s). In comparative experiments, members of the complementary group, the control group, receive either no treatment or a standard treatment.
For the conclusions drawn from the results of an experiment to have validity, it is essential that the items or patients assigned to treatment and control groups be representative of the same population. In some experiments, such as many in agriculture or psychology, this can be achieved by randomly assigning items from a common population to one of the treatment and control groups. In studies of twins involving just one treatment group and a control group, it is statistically efficient to do this random assignment separately for each pair of twins, so that one is in the treatment group and one in the control group.
In some medical studies, where it may be unethical not to treat patients who present with symptoms, controls may be given a standard treatment. Another alternative is to select controls from a wider population, provided that this population is well-defined and that those presenting with symptoms at the clinic are representative of those in the wider population.