What is a control variable?

• A. A variable kept constant to avoid affecting the outcome.
• B. The variable deliberately changed to test effects.
• C. The outcome being measured.
• D. The random sampling method.

Answer: (A)

In experiments, a control variable is a factor kept constant so it doesn’t influence the result. For example, if you’re testing how fertilizer affects plant growth, you’d keep light, water, soil type, and pot size the same for all plants. The thing you actually change is the fertilizer amount (the independent variable), and the outcome you measure is plant height or biomass (the dependent variable). By keeping other factors constant, you can be confident that any difference in growth comes from the fertilizer, not from those other variables.

The other ideas aren’t the same: changing a variable to test its effect is the independent variable, not a control. The outcome being measured is the dependent variable, not a constant factor. The random sampling method describes how you select subjects, not what you hold constant during the experiment.