In experimental design, what is the primary function of a control group?

• A. Minimizes Type II error
• B. Baseline for controlling the effects of the manipulated variable
• C. Increases external validity
• D. Reduces required sample size

Answer: (B)

A control group provides a baseline to isolate the effect of the manipulated variable. By keeping all conditions the same except for the variable being tested, the control group lets you see what happens without the experimental treatment and then compare it to what happens when the treatment is applied. This helps ensure that any observed differences are due to the manipulated variable and not other factors like natural variation, placebo effects, or participant expectations, thereby strengthening internal validity.

The other ideas aren’t the primary function: minimizing Type II error relates to statistical power and detecting real effects, which depends on sample size and variance rather than the role of a control group. Increasing external validity concerns generalizability beyond the study setup, which isn’t achieved just by having a control group. Reducing the required sample size isn’t the goal of including a control group; in fact, controls can require more participants to maintain reliable comparisons.