A variable that remains unchanged or held constant to prevent its effects on the outcome and therefore may verify the behavior of and the relationship between independent and dependent variables. Supplement. Control variables are important in scientific experiments to test the validity of the results.For instance, in an experiment on Boyle's law, the temperature must be kept constant so that the outcome from the experiment can be regarded as valid.ontrol variables are important in scientific experiments to test the validity of the results. For instance, in an experiment on Boyle's law, the temperature must be kept constant so that the outcome from the experiment can be regarded as valid.