Controlled Variable Definition (Control in an Experiment)

What Is 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 (not the independent or dependent variable), but it is important because it can have an effect on the results. It is not the same thing as a control group.

Any given experiment has numerous control variables. 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 variable. 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 the amount of light, always using the same type of glassware, constant humidity, or duration of an experiment.

Common Mis-Spelling: controled variable

Importance of the Control Variables

Although control variables may not be measured (although 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". Noting control variables makes it easier to reproduce an experiment and to 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.

Learn More

  • What Is a Variable?: A control variable is only one of the variables in an experiment. Learn about the different types.
  • What Is a Controlled Experiment?: An experiment may be controlled or uncontrolled (even if it contains a control variable). Here's how to tell them apart.