Sometimes in experimental research, a pretested experimental group may be used. When using a pretested experimental group, a determination has been made, before the experiment, to indicate a performance level for the experimental group.
An un-pretested experimental group design is used when the researcher does not pretest the experimental group. The performance level of an un-pretested experimental group is unknown prior to exposure to the treatment.
A control group in experimental research is a group that does not receive the treatment. It is used to determine if not receiving the treatment provides the same effect as receiving the treatment. A positive control group is sometimes used when testing a treatment known to have a positive effect.
Sometimes a research may choose to use a negative control group in his experimental design. A negative control group dose not receive any treatment and is not expected to produced the desired effect.