Groups of subjects being compared to one another should be as a similar as possible. For example, one would not want to apply an experimental treatment to a group of first graders and use a group of second graders as the control. In order to make valid comparisons between treatments, it is important the therapies of the groups are fairly compared. If not, the experimental therapy could appear superior to the control, even though it is not. This is a particular concern with investigations that have a commercial sponsor. Hypothetically, a study could be conducted to determine if plaque removal with a new mouthwash and tooth brushing (experimental group) is superior to removal with a toothbrush alone (control group). If the control group is given less time for brushing than the experimental group, or if only the experimental group is allowed to use toothpaste, the control therapy is being set up to show less effectiveness. The procedures used by the groups should be comparable and described clearly in the Methods section so the reader can make these judgments.
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