The independent variable is "controlled" or held constant in the control group. A single experiment may include multiple experimental groups, which may all be compared against the control group. The purpose of having a control is to rule out other factors which may influence the results of an experiment.
A control group is a standard group which is used for the purpose of the experiment. It does not receive any manual treatment by the experimenter. Instead it is kept as a standard for comparison of the other variables.
The multiple control groups are used in the experiment so as to increase the accuracy, consistency and reliability of the results. As if one control group contains contaminants it may produce errors in the experiment. To avoid error and enhance error and increase accuracy multiple control groups are used.
