Controls, or "control subjects" are individuals who participate in a research study and receive a standard treatment to be compared with the experimental treatment.

For example, in a research study to evaluate whether a new drug improves memory, researchers may assign the subjects randomly into two groups, the experimental group and the control group. Subjects in the experimental group will be given the new drug, while subjects in the control group will be given an existing drug (or perhaps a placebo). Later, the researchers will give a memory test to all subjects. If the subjects in given the new drug perform better than control subjects, this will be evidence that the new drug is effective. On the other hand, if subjects given the new drug perform no better (or even worse) than control subjects, this will be evidence that the new drug is ineffective or no better than existing treatments.