In biology and all other forms of science and mathematics, a controlled variable is one that is completely controlled throughout an experiment. Controlled variables stay constant throughout experiments and are carefully controlled and observed.
Controlled variables are controlled to observe the relationship between the non-controlled variables in the experiment. Independent and dependent variables are non-controlled variables in experiments. These variables may have a change in behavior whenever a controlled variable enters the experiment. Scientists and mathematicians utilize controlled variables to observe changes in behavior in non-controlled variables and to determine if a controlled variable can change the outcome of the entire experiment.