What is the purpose of a control group in a science experiment?

• A. To change the variables
• B. To compare results while keeping other variables the same
• C. To speed up the experiment
• D. To measure only one result

Answer: (B)

A control group provides a baseline for comparison by keeping all factors the same except for the one variable you’re testing. This helps you see exactly what effect the independent variable has, because any differences between the control and the experimental group can be linked to that variable rather than to other factors. For example, if you’re testing how a new fertilizer affects plant growth, the control plant would get the usual care and environment, while the plant with the fertilizer would be the only one with the changed variable. Everything else—light, soil, water—stays the same.

This idea is why the answer is best: it emphasizes comparing results while keeping other variables constant so you can attribute observed changes to the variable you changed. The other options aren’t the purpose: changing variables would blur cause and effect, speeding up isn’t what a control group does, and measuring only one result isn’t about the role of a control group.