The Role of a Controlled Variable in an Experiment

If all parts of an experiment are conducted at the same temperature, then temperature is a controlled variable.
If all parts of an experiment are conducted at the same temperature, then temperature is a controlled variable. Level1studio / Getty Images

A controlled variable is one which the researcher holds constant (controls) during an experiment. It is also known as a constant variable or simply as a "control." The control variable is not part of an experiment itself—it is neither the independent nor dependent variable—but it is important because it can have an effect on the results. It is not the same as a control group.

Any given experiment has numerous control variables, and it's important for a scientist to try to hold all variables constant except for the independent variable. If a control variable changes during an experiment, it may invalidate the correlation between the dependent and independent variables. When possible, control variables should be identified, measured, and recorded.

Examples of Controlled Variables

Temperature is a common type of controlled variable. If a temperature is held constant during an experiment, it is controlled.

Other examples of controlled variables could be an amount of light, using the same type of glassware, constant humidity, or duration of an experiment.

Importance of Controlled Variables

Although control variables may not be measured (though they are often recorded), they can have a significant effect on the outcome of an experiment. Lack of awareness of control variables can lead to faulty results or what are called "confounding variables." Additionally, noting control variables makes it easier to reproduce an experiment and establish the relationship between the independent and dependent variables.

For example, say you are trying to determine whether a particular fertilizer has an effect on plant growth. The independent variable is the presence or absence of the fertilizer, while the dependent variable is the height of the plant or rate of growth. If you don't control the amount of light (e.g., you perform part of the experiment in the summer and part during the winter), you may skew your results.