Controlled experiments are performed under controlled laboratory conditions. There are set of conditions that influence the type of result. It is based on these conditions that the results can vary. Such conditions that can be manipulated are called variables. Variables are of three types; controlled variables, manipulated variables and respondent variables. These variables are dependent on each other.
If a biologist wishes to study the effect of a single variable, he has to change only one variable at a time and keep the other variable constant. Control variables are set of conditions, where in the experimental conditions are kept constant without any manipulations.