Which statement best defines the independent variable?

• A. The variable that is manipulated
• B. The variable that is measured
• C. The variable that is randomized
• D. The sample population

Answer: (A)

The independent variable is the factor you deliberately change to test its effect on another outcome. It represents the cause in the cause‑and‑effect relationship being studied, and you set its different levels to observe what happens to the outcome you measure. For example, in a study on caffeine and reaction time, the amount of caffeine given is the independent variable, while reaction time is what you measure as the dependent variable. Randomization describes how participants are assigned to groups to reduce bias, not a variable you manipulate. The sample population is the group you study, not a manipulated factor.