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The main idea here is keeping factors constant to see what the manipulated factor actually does. A variable that is intentionally held constant across all parts of an experiment is a control variable. By keeping this factor the same, it can’t influence the outcome, so any observed change can be attributed to the factor you deliberately changed—the independent variable. For example, in testing how fertilizer affects plant height, you would keep sunlight, soil type, and watering amount constant to ensure that differences in height come from the fertilizer, not from those other factors.

This is different from the independent variable, which is what you deliberately change, and from the dependent variable, which is what you measure. An extraneous variable is any unintended factor that could affect the results; it’s not something you intentionally hold constant across conditions—you try to control it, but it isn’t defined as the factor kept constant for the purpose of isolating the effect of the manipulated variable.