A control group is set aside in an experiment specifically to counter:

1. Subjects who know they are being treated may improve for reasons unrelated to the treatment. 2. To counter the power of suggestion, researchers set aside one treatment group as a control group. 3. This group is given a placebo treatment, an active treatment that cannot directly influence the response variable. 4. Comparing against that group isolates the treatment's real effect. 5. Cost and sample size are practical concerns rather than the reason for a control group. _Source: OpenStax Introductory Statistics 2e (CC BY 4.0), Ch 1 'Sampling and Data', sections 1.1-1.4_