Definition · Plain-language
Control group
A control group is the set of participants who do not receive the experimental treatment, providing a baseline against which the treated group is compared.
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The baseline for comparison
An experiment compares at least two groups that are treated identically except for the independent variable. The experimental, or treatment, group receives the intervention; the control group does not. Because the two groups differ only in whether they received the treatment, any difference in the outcome can reasonably be attributed to the treatment itself. The control group answers the essential question "compared with what?" — it shows what happens in the absence of the intervention, so improvements that would have occurred anyway (through natural recovery, practice or the passage of time) are not mistaken for treatment effects.
Placebos and blinding
In studies of people, simply knowing you have received a treatment can change the outcome — the placebo effect. To separate the treatment’s real effect from this expectation, the control group is often given a placebo: an inert substitute, such as a sugar pill, that is indistinguishable from the real intervention. Blinding strengthens this further. In a single-blind study participants do not know which group they are in; in a double-blind study neither participants nor the researchers assessing outcomes know. Blinding prevents expectations and observer bias from contaminating the comparison.
Randomised controlled trials
The randomised controlled trial (RCT) is the design that combines a control group with random assignment, and it is widely regarded as the gold standard for testing whether an intervention works. Randomly allocating participants to the treatment or control group ensures the groups are comparable at the outset, so confounding variables are distributed by chance rather than systematically. Control groups take several forms: a no-treatment control, a placebo control, or an active control that receives the current standard treatment, used when withholding any treatment would be unethical.
Key facts
At a glance
- Definition: the group that does not receive the experimental treatment
- Purpose: provides a baseline for comparison with the treatment group
- Contrast: the experimental (treatment) group receives the intervention
- Placebo: an inert substitute given to control for the placebo effect
- Gold standard: the randomised controlled trial pairs control with randomisation
- Variants: no-treatment, placebo and active (standard-care) controls
Common misconceptions
What people often get wrong
Often heard: The control group receives no attention or interaction at all during the study.
Actually: The control group is treated identically to the treatment group in every way except the intervention. It may receive a placebo or standard care; what defines it is the absence of the specific treatment being tested.
Often heard: A control group is optional as long as you measure the treatment group before and after.
Actually: A before-and-after measure cannot rule out natural change, practice effects or the passage of time. Only a comparison with a control group shows whether the treatment caused the difference.
Often heard: It is always unethical to give a control group a placebo instead of real treatment.
Actually: When an effective standard treatment exists, an active control receiving that standard is used instead of a placebo. Placebo controls are appropriate mainly when no proven treatment is being withheld.
