CONTROL GROUP

What is a control group? In an intervention trial to examine whether an intervention has an appreciable effect, a control group is used as a precaution against the possibility that change would have happened even without the intervention. If participants can be randomly assigned to a no-intervention control group, then if more positive change happens in the control group than the intervention group, the difference is highly likely to be due to the intervention, and not due to other factors.

Why do participants have to be randomly assigned to a control group?

With large samples, random assignment to intervention and control groups should help to ensure that there are no systematic differences between the two groups. If you have a smaller sample, you should be wary of random assignment creating different samples by chance, and it may be better to use MATCHED SAMPLES. 

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