Control Group

The control group is an important part of the design and conduct of clinical trials and experiments. The control group is the group that gets either no treatment, a fake treatment, a placebo or the current standard treatment. The treatment group (or experimental group) is the group that gets the treatment or intervention that is being tested (the independent variable).

The control group is important so that the placebo effect and natural history of the condition is controlled for in the study. The purpose is to demonstrate that the treatment in the treatment group is better than a placebo or natural changes that happen in the disease progress over time (eg regression to the mean).
For example, some conditions improve or change over time naturally, so if a treatment is assumed to work in clinical practice, did it really work or did the patients just improve naturally? The use of a control group in a clinical trail is to account for this natural history.

An ideal control group is:

  • identical in all of the known characteristics as the treatment group (the baseline measurements between the two groups are the same before the treatment or intervention is given to the treatment group).
  • allocated to the control group and treatment group by proper randomization (so it can be assumed that they are the same in the unknown characteristics as the treatment group).
  • participants do not know which group they are in until after the study has been completed and the follow up measurements done (ie blinding). Participants allocated to the control group need to at least believe that they are in the treatment group.
  • researchers who are doing the follow up assessments do not know which group the participant is in until after they have done the final assessments or measurements (ie blinding).

As researchers change the independent variable in the treatment group and keep it constant in the control group, they then compare the differences between the two groups. This way any change in the dependent variable can be attributed to the effect independent variable (the treatment or intervention).

Some issues in studies that used a “control” group:

  • there were differences in some characteristics between the two groups at baseline before the treatment or intervention is given (ie they were different in the dependent variable before the independent variable was applied).
  • they were not allocated to the group by appropriate randomization.
  • the results were analyzed by a within groups analysis when they should be analyzed by comparing the differences between the treatment and control groups (ie a between groups analysis).
  • participants or researchers doing the assessments were not blinded to the group that were in.

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