What is control group?
control group is A group of participants that serves as the basis of comparison when assessing the effects of the intervention of interest that is given to the participants in the treatment group. Depending upon the circumstances of the trial, a control group may receive no treatment, a ‘usual’ or ‘standard’ treatment, or a placebo. To make the comparison valid, the composition of the control group should resemble that of the treatment group as closely as possible.
source: A GLOSSARY OF TERMS FOR COMMUNITY HEALTH CARE AND SERVICES FOR OLDER PERSONS. World Health Organization, 2004.