Control groups are used in controlled experiments to curb bias. Both groups are treated exactly the same in every way possible except for variable difference. The control group and experimental group must be identical in all relevant ways except for the introduction of a suspected causal agent into the experimental group.
Control groups are not manipulated. They are used to compare the experimental group against. The control group is used so that you have something to compare the experimental group to. Control groups, double blind procedure and randomized testing are used to reduce error, self-deception and bias.
We can test plant fertilizer by giving the fertilizer to only fifty percent of the plants in a nursery. The plants that don't receive the fertilizer are the control group, as they establish the baseline growth level that the fertilizer-treated plants will be compared against. Without a control group, we cannot determine whether the fertilizer-treated plants could have grown more than they would have if they did not receive the fertilizer.
Control group is a group of people who serve as a standard or reference for comparison with an experimental group. A control group is identical to the experimental group in number and is identical in specified characteristics, such as sex, age or other factors, but does not receive the experimental treatment.
Double blind procedure is a method of enhancing internal validity in an experiment. In double blind procedure, neither the researcher nor the subjects are made aware of which group is the experimental group and which the control group.