The control variable, also known as the constant variable, is the variable that remains unchanged throughout an experiment. Control variables minimize room for errors as they reduce the factors that can affect the outcome of an experiment.
In an experiment designed to show how the amount of light affects growth in plants, the type of light involved could be a control variable. Other possible control variables in the same experiment are the type of plant tested, the container used, the type of soil used, the temperature, the humidity and the amount of water used.
Apart from control variables, independent variables and dependent variables also factor in experiments. The independent variable is the condition that changes during an experiment. For example, in an experiment designed to study the growth of coffee beans, some independent variables may include the amount of fertilizer used, the amount of water used and the temperature.
The dependent variable is the variable that the scientist observes or measures. For example, in an experiment tailored to investigate the effect of temperature change on solubility, solubility is the dependent variable. In the experiment to show how plants grow in response to certain amounts of light, plant growth is the dependent variable.