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Control Group

A bottle of pills - a dummy treatment.Intervention projects or experiments should always compare treatments or interventions rather than try to assess effectiveness in isolation. 

Semantics or Science?

What would you think if I told you 54% of patients that received a gastric freezing treatment for ulcers improved in condition? Sounds like a fairly impressive treatment.

BUT, what if I also told you 56% of patients with ulcers that received a dummy treatment also improved? The freezing treatment doesn't seem so impressive anymore.

What is a control group?

The group that received the dummy treatment is called a control group, because it allows us to control for the effects of variables that have an influence on the response. In this example, by using a control group, we are able to see that patients seem to get better about 55% of the time regardless of whether or not they are treated with gastric freezing. 

If we had looked at this treatment in isolation, without a control group to compare it to, we would have mistakenly concluded that gastric freezing is an effective treatment. 

How to Select Your Treatment and Control Groups

Participants in an intervention or experiment should be randomly assigned to either the treatment or control group. This can be accomplished a number of ways. Commonly, people use random digit tables or statistical software to randomly assign the treatment groups to subjects. Random assignment reduces the biases in your groups.Because random assignment leaves the decision of which group an individual participates in to chance, it ensures there is no bias in how the groups are created and unmeasured factors which affect your outcome are distributed equally between your intervention and control group. 


Suppose a researcher wishes to assess a nutritional supplement’s claim to help weight lifters gain weight faster than simply consuming the same number of calories. 

In order to test this claim, the researcher decides (s)he will recruit 30 subjects and divide them into 2 groups. The treatment group will receive the nutritional supplement; the control group will receive food with an equal amount of calories as the supplement. 

To randomize the subjects into the two groups, the researcher assigns a number 1-30 to each subject. We decide to use software to return 15 random numbers between 1-30. We get the following numbers:

  01   07   05   02   13   10   27   19   06   23   25   09   12   11   04

All of the subjects assigned these numbers will be in the treatment group receiving the nutritional supplement. All other subjects will be in the control group.





rev. 04-Aug-2022




Resource Library

Find links to research conducted by members of NEDARC. Review their methods for selecting control groups.


Call your NEDARC state/territory contact for help in setting up a control group.


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