What is a control variable and why is it important in experiments?

• A. The factor kept the same across trials
• B. The variable measured
• C. The device used to measure
• D. The time of day

Answer: (A)

A control variable is a factor that stays the same across all trials. Keeping it constant is essential because it prevents other differences from affecting the outcome, so you can tell whether the change you make to the independent variable really causes any observed effect. For example, if you’re testing how sunlight affects plant growth, you’d keep soil type, pot size, water amount, and room temperature the same. The thing you actually measure—the plant’s height or growth over time—is the dependent variable. The device you use to measure, like a ruler, is just the instrument. Time of day isn’t the defining control variable, but it could introduce variation if not kept consistent in some setups. By controlling these extra factors, the results reflect the impact of the variable you’re testing rather than other influences.