What is the primary purpose of a control group in a scientific experiment?

The primary purpose of a control group in a scientific experiment is to compare results to a baseline or standard. The control group receives no treatment or a standard/placebo treatment, while the experimental group receives the variable being tested. By comparing the outcomes of both groups, the researcher can determine whether the independent variable caused any observed changes.

Without a control group, there is no reference point. Any changes observed in the experimental group could be due to the treatment, natural variation, placebo effects, or external factors. The control group isolates the effect of the independent variable by keeping all other conditions the same.

The other options describe different aspects of experimental design. Reducing experimental error is achieved through techniques like randomisation, replication, and blinding, not specifically through having a control group. Testing the effect of multiple variables relates to factorial experimental designs, not the role of a control. Increasing sample size simply means using more participants, which improves statistical power but is unrelated to the specific function of a control group.