What characteristic defines a control group in an experiment?

The defining characteristic of a control group in an experiment is that they receive no treatment. This group serves as a baseline for comparison against the experimental group, which does receive the treatment or intervention being studied. By not receiving any treatment, the control group helps researchers discern the true effects of the treatment by allowing them to observe what happens in the absence of that treatment. This helps eliminate confounding variables and biases, providing a clearer understanding of the treatment's actual impact.

While it is also common for a control group to receive a placebo, this is not a universal requirement. A placebo is used to simulate the treatment experience without providing the actual treatment's effects, but the essence of a control group is its lack of treatment altogether. The other options involve aspects of either the experimental process or the subjects' characteristics, but they do not capture the fundamental principle that distinguishes a control group.